Nope, I'm sorry but it won't work and isn't safe for the baby nutrition-wise...If you're going to pull a baby bird from the nest-box and hand-feed them formula, you absolutely must pull them between 2-3 weeks old without question. Period. Any earlier and they don't get the immune-system building-blocks that they need from their parent's "crop-milk", and any older than 3 weeks causes a host of different issue, but the biggest issue is that after 3 weeks old it's pretty much pointless to pull them or hand-feed them in the first place, as they are too old to accept the hand-feeding formula..I have seen people try to pull babies at 4 weeks old, and even at only a week late it was extremely difficult and the end-result was a baby bird who had a horribly tough time abundance-weaning.
***You have to realize that Cockatiels should start abundance-weaning between 5-6 weeks old! So obviously you can't pull them from the nest-box at 5, 6, or 7 weeks old to hand-feed them formula, as not only will the baby refuse the formula completely and not allow you to feed them, but by the time you would be pulling them to hand-feed them, they should already be living in their Weaning/Starter Cage and be well into eating solid foods...To put it into perspective for you, a baby Cockatiel who is 5 weeks old should already be down to 3-4 hand-feedings a day, at 6 weeks old they should be down to 2-3 hand feedings a day, and at 7 weeks old we're talking them pretty much only getting a feeding first thing in the morning and then again at night right before bed. And by the time a baby Cockatiel is already between 5-7 weeks old, they've already been strictly fed by their mother and father for that long a period of time, and they absolutely will not accept the hand-feeding formula, or the syringe, nor you feeding them, and this can cause a huge behavioral issue for them, and can create a serious problem getting them to properly abundance-wean, which is the absolute last thing you want to have happen, as the typical results are very serious and life-long neurological issues...
It's extremely difficult for most people to be able to hand-raise/hand-feed baby birds, simply due to the necessary feeding schedule, and it's exactly the reason that most die-hard parrot people do not undertake breeding birds, or if they do start breeding birds it's exactly the reason that they allow their babies to be parent-raised/parent-fed.
To be able to hand-feed a baby parrot, regardless of the species of parrot, you must be able to pull them from the nest-box between the ages of 2-3 weeks old, and at 2 weeks old you must be able to hand-feed them once every 2-3 hours, then at 3 weeks once every 3-4 hours, and the same at 4 weeks old, once every 3-4 hours, AND up until this point this includes feeding them every 3-4 hours OVERNIGHT. Only at 5 weeks old can a baby bird go for 6-8 hours overnight without eating, and at 5 weeks they need to be fed once every 4-5 hours. And then depending on the species of parrot, this is the time when you start the "Abundance-Weaning" process, where the baby is moved from the Brooder to their "Weaning/Starter-Cage" and start actually providing them with a bowl of pellets, a bowl of seed-mix, and then also fresh veggies and fruit. For a Cockatiel, they are typically fully Abundance-Weaned between 8-10 weeks old, sometimes longer, sometimes sooner...So you can see the problems you would have in pulling them later than 2-3 weeks old, and I'm pretty much totally ignoring the fact that the baby bird is not at all going to be happy with you trying to feed them after 5+ weeks of them being fed by their parents, and this is why they will pretty much always refuse all hand-feeding formula/feedings by a person after they are at the very, very latest 4 weeks old, and again, even at 4 weeks old it can be too late..
***The best thing that you can do is forget about hand-feeding your baby Cockatiel completely, as hand-feeding a baby bird has absolutely no bearing at all on how tame the bird will be, how strong a bond you will have with the bird, etc. Frankly that's all a bunch of total BS, and any experienced bird-breeder will totally and completely agree with me. All you need to do is allow the baby to stay in the nest-box and allow the parents to raise and feed the baby completely, and all that you need to do in order to start the hand-taming process and the bonding process with this baby is to make sure that you are taking the baby out of the nest-box several times a day, handling him several times a day, talking to him several times a day, etc. And the older the baby gets the more time you should be spending with him one-on-one, handling him, playing with him, cuddling with him, etc. If you do it this way you will form just as close a bond with the baby as you would by hand-feeding him from 2 weeks old. And this absolutely will not make the parents "reject the baby", another bunch of BS. You can touch the baby, handle the baby, etc., and as long as you return the baby to the nest-box so that he can fed on-time by his parents, and as long as you never force the parents out of the nest-box while they are laying on him or feeding him, you'll be fine and they'll be fine. Just wait for both parents to leave the nest-box, and then immediately block-off the nest-box hole so that the parents cannot get back into the nest-box and leave it blocked-off until you return the baby back to the nest-box. Then once you return the baby you can unblock the nest-block and allow the parents back in.
And then once the baby comes out of the nest-box on his own and is eating pellets and seed on his own you can spend much more time with him outside of the room where the parents are, as at that point they are typically only getting a feeding first thing in the morning and then a "comfort-feeding" right before bed (please be sure to provide pellets to the baby the minute he comes out of the nest box so that he'll wean-onto them and not just seed). And make sure that you have all different kinds of toys and foraging activities inside of the cage that the nest-box is in and the parents live in, at least 6-10 different toys, lots of different types of perches made of different materials, different types of food and water bowls/bottles/dishes, etc., as Cockatiels are extremely prone to actually not "learning how to play with toys" if the breeder does not ensure that there are a ton of toys inside of the cage BEFORE the babies come out of the nest box and start weaning.
I bred Budgies, Cockatiels, and several species of Conures for over 20 years, and yes, I hand-fed most of the babies I bred, pulling them all between 2-3 weeks old, running on only 3 hours of sleep each night and then going to work, and running home from work every 3 hours to feed them...I was lucky in that I have worked in the car-business for the past 15 years, since the end of 2003, so I have great flexibility to pretty much come and go from work as I please, as in the car business, if you're in sales, you are running your own business and you only make what you make, no salary or hourly-rate. So this job worked extremely well with breeding and hand-raising birds. And prior to that i was in college and then grad school, so that allowed me the same flexibility. But most people do not have this luxury...The good news is that it really doesn't make any difference at all, just as long as you are handling the baby, talking to the baby, and spending time with the baby every single day and allowing his parent-birds to be responsible for the feeding and housing/warmth part, you are basically "co-parenting" with the parent birds and you will not only hand-tame the baby just the same as would happen if you had hand-fed him, but you will also bond every bit as closely with him. And he'll get all of the nutrition he needs, he'll properly abundance-wean, he'll have no behavior, psychological, or neurological issues because you tried to actually take him away from his parents and start hand-feeding him after he's already been raised and fed by his parents for 5+ weeks, etc.
It's extremely important that you do what is right for the physical and psychological health and well-being of the baby bird in this situation...
