caribes are nice, piraya is very colorful as well. they both require bigger tanks than rbps but they won't get to that size over night. you just need to know what you are getting into with the need of upgrade to a bigger tank few years down the road or have to sell them off.
caribes are not in season now so it'll be hard to find them in small sizes, the smallest I've seen still available is around 5-6". only 3 left in Shark Aquarium and you need to buy all three, I think the asking price is $200.
should have small ones (1-2") available during summer (I got mine in June) so you can go for a caribe only tank by sell off your two rbps. that's what I would do.