I had mine for more than five years with: Oscar, silver dollars, various catfish, mollies, guppies, swordtails parrot fish and plecos. (obviously not all groups at the same time)
The aggresiveness depends on the individual, its best not to have him with another one of its kind (or any shark-kind), as they will establish territories and they will either chase the other around or kill each other.
Bottom line is: Think rainbow sharks as a betta and you wont go far. Their personalities and behaviours are too similar. Also keep the lid closed, i learned that the hard way. Oh and remember they need three things: Space, space, space.