A lot of people will say that you shouldn't keep any other sharks with rainbows, but I think it can work and have seen it work if the tank is large enough with plenty of hidey spaces. As for the shark, mine is quite happy eating almost anything I give him. They need algae/veggie matter so make sure to supplement if there isn't enough algae growing in your tank. I also have found, at least with mine, that he gets stressed and loses fin color if the PH is lower than around 7.0.
Aside from all that, I have had my albino rainbow longer than any other fish and he(she?) has been very easy to deal with.
Good Luck!