Re: New instructor on the block
Just my own personal opinion, but I would try very hard NOT give a student extra attention if they seemed quicker (or slower) to pick up new concepts. If a student is working hard and really giving the training their attention, IMO they deserve your attention, period. Besides, being quick to roll on the first day probably just means they're in good physical shape and not fearful... it might or might not mean anything in the long run. I would probably just congratulate them and tell them to practice it a lot until it gets more automatic. But I don't see what anyone would gain by making a big deal of it or comparing students.
As far as joking around, you could joke around while you're training and not while you're teaching - people will figure it out. Or you could joke around while you're teaching, too. Depends what works well for you and if your Sensei has any preferences about teaching style.
As I say, that's purely my opinion...