Congrats on the new baby Tiel...Although I'm a bit concerned if he's "really small" and has no feathers on his head yet? But he's eating solid food? But he can't fly well? It sounds to me like he was weaned onto millet far too soon...I've seen that before, as when you're hand-feeding a baby bird, they will start to eat millet sprays while they're still being fed multiple feedings of formula a day...And then people go "Oh, they're eating the millet seed, so we can stop the formula"...That's not good, so I'm glad you've got him now...Make sure you let his wings grow-in fully and don't clip them again, he needs to be fully-fledged and be able to fly with total control first.
As far as his diet goes, YES he can eat fresh veggies and fruit, that's ridiculous. Most breeders/hand-feeders start giving their babies fresh veggies and fruits while they're weaning, as that's the only way they'll ever learn to eat them!!! Bad information. Please, go ahead and start feeding him plenty of fresh veggies, dark leafy greens, and a little fresh fruit each day...
As far as the "millet" diet, what I would do immediately is go and buy a healthy, low-fat seed-mix with no sunflower seeds, no nuts/peanuts, and no dried corn, one such as Tropimix or Higgins California Blend, or even Higgins Safflower Gold...He should start eating those right away, and combined with lots of fresh veggies, greens, and fruit, then his diet is actually pretty good, and you can do it immediately...Then worry later about transitioning him onto a pellet staple diet if that's what you're going to do...But I think it's much more important to eliminate all that fat from his diet right away so his liver isn't effected at such a young age...And seed-mixes such as Tropimix (which also contains Tropican pellets in it too), Higgins California Blend, etc. are very healthy and varied seed-mixes with tons of healthier, low-fat seeds, fresh grains, legumes, herbs, veggies, fruit, etc. and none of the bad stuff or the multiple different kinds of millet and that's it. Get him off on the right start, because I think he's had a rough beginning with his wings clipped already and them weaning him a bit too early and onto crappy millet seed and no veggies or greens...
As far as introducing him to your IRN, as someone who has 5 different parrot species living together plus a Ringneck Dove, you just have to gradually let them spend supervised time together, making sure that your IRN can't get a hold of him with his beak (Cockatiels don't tend to be very aggressive, and the beak-size difference is massive)...It's all up to them whether they will get along, not you. It might happen, it might not, and you cannot force it.
After the quarantine I would simply put their cages into the same room with each other, but not right next to each other, so that they can see each other and talk to each other, but not so close that either will feel anxious about it...Then after a couple of weeks of having them out at different times you can move their cages next to each other and let them be close for a while, and again, when you decide to actually let them out together for the first time you must have total control over your IRN...And just see how they do...There is no way to know whether they'll get along or not...Sometimes they'll just tolerate each other, sometimes they'll really like each other, sometimes they'll hate each other, sometimes they'll want to kill each other and won't be able to be together at all...It just depends on them...but introducing them slowly and putting their cages together in the same room for a while to let them get to know each other is the best way to start after the quarantine.
. They will grow back in though! 
really? I wanted to double check so I asked! 
