He's such a cute guy, was really well behaved in the pet store, sitting so well on my shoulder AND finger. Now at home he's a pretty rough biter?
"Normal" behavior. Birds can become scared and nervous in new environments and may bite to show that they aren't comfortable. You also have a baby... Basically, a feathered puppy with a can opener for a mouth! Puppies *LOVE* to get into everything! Well, your baby is communicating through bites and possibly because he is curious, too!
He's picked me to be his absolute favourite tree! However he bites my hand/finger?
It may help to figure out if he's biting out of fear/communication or boredom. If it's boredom, give him something to do! If it's the former, what is it he's trying to say?
if I don't let him on my shoulder he will click, flap and squeek for my attention. Regardless if I put him on a perch or in cage.
It's a great time to start teaching independent play and foraging activities!

The more you engage his mind in self play, the better well rounded he'll become as an adult!
He basically can't come off me because he can't step up and down yet...question is...should i let him be on my shoulder?
The options are.... either he doesn't want to, or he doesn't know what you are asking of him. Try some target training through the cage bars and reward him with his favorite treats. Once he's kind of picking up on that, start some target training around the outside of the cage. This can then be used to teach him to come down off your shoulder, even if it means literally walking 'down' your arm for a reward!
Are we having a connection or am I just a tree? My partner is 6.4 and he's a better tree but even on him Conure just flaps to get me to come over.
He(?) may have been raised by a women, so has a natural preference for women. Definitely get your partner interacting with him and and training too! It's healthier if he doesn't bond too strongly to one person or another.
He's cute but bites aren't okay! Also he's with us 1.5 days and I think for that he's just great
he doesn't want to poop on me too which is nice! How nippy is okay and should I let him on my shoulder?
IMO, any amount of nipping can be bad nipping. This can lead to bites, bites can lead to aggression or worse. Do your utmost best to avoid bites by learning to read your bird's body language, redirecting into desired behaviors and training new behaviors! If you do get bit, don't punish and don't ignore! Simply pry his beak off of you and set him down for a few moments! That's really all that's necessary! Give yourself some time to figure out what went wrong and how you can avoid that situation in the future.
As far as the shoulder goes? Completely up to you! I allow my birds on my shoulders and head if they want. Some people aren't comfortable with it because it's harder to read your bird's body language and they can cause serious damage to our faces!