Hi and welcome!
And of course, welcome to the world of senegals, where sometimes they bite like a bulldog for absolutely no reason. Or is it?
Try to think back what exactly were you doing that you've gotten bitten. Were you opening an envelope? Did you just lean on the kitchen counter? Talking on the phone? Drinking coffee, scratching your nose?
Senegals are amazing little stocky parrots, with an incredibly strong beak which bite will hurt for the next week. They are very inteligent, funny clowns, and some are true cuddle bugs. However, they have this particular trait where they are extremely sudden and fast, and will atack and bite in an instant. I can tell you there is always a reason, at least in the senegal's eyes. It can be as dumb as just looking them the wrong way, blowing your nose, or taking your medicine. Try to find out what triggers him, and try your best to avoid that action right in front of the bird. Also many times they seem to bite out of fear - if they don't want to be touched, they will surely let you know. And you will wear a bandage for a few days.
Sudden movements can also trigger them. I'd like to ask, how old is the bird? They are known with their terrible two's, when they hit sexual maturity they are true little monsters that bite and attack relentlessly. I know that my partner's Senegal was that way, for a full year, and was almost rehomed. She did leave him a handful of scars on his hands and ear. Thankfully, she changed and calmed down, and is the sweetest little thing I have ever come across, but nevertheless she still has her cobra reflexes and bites. Often what happens is that her reflexes are stronger than her, and she simply knows she wasn't suposed to bite, and she will act all cute and apologetic. This happens during playtime, and she did bit my finger here and there quite strong, leaving me with a bruise for a week. If the bird is not apologetic, I probably deserved it.
In our case, no rustling with papers nor plastic bags, no packaging rustling in front of her, no pill bottles or small boxes which contain medicines, absolutely no touching when she's sitting on the chair, NO touching while she's preening, no sudden movements or arm flailing... Or trying to do a photo too close. Try to carry the bird into another room once you know there is a certain action or item that triggers him. You can also try to do it slowly and see how the bird reacts. If the bird was on the edge but didn't attack, praise and give treats! However, this is a really difficult task, and don't be desparate if the bird simply never learns not to be triggered by something.
Now onto the previous owners - what is the reason of rehoming? Keeping a bird for less than 6 months is weird, but not necessarily a huge alarm if the bird is good natured.
As for the diet, we personally feed Harrison's pellets, bit of seed mixe (safflower, milkthistle, tiny bit of sunflower as a daily treat...), Her usual veggies and fruits, and the boiled legume mix. The pellet diet has become lately more of a personal choice it seems, but if the bird is very active, if it is healthy and eating a great variety of seeds (not just millet and sunflower..) along with his veggies and some fruits, all should be fine. If the bird is a couch potato and refuses to eat more varieted seed mixes, and will pick out a lot, then perhaps a pelleted diet would be a better choice, or at least as a supplement.