Depends on how many fish you are going to keep in there, but in a 90 gallon I believe that they will come pretty close to full grown (10-12 inches maybe). I don't think any fish kept in aquariums reach full size, but they do come very close. So I would think with the exception of a gigantic piranha in the wild you won't really even notice the differance
Yes it very much depends on how many fish you have in there. 10 caribe in a 90 would not get as large as 3 in a 90. All depends on room per fish. The more room (less fish) the larger they should potentially get.