Kribs are great.
They are super passive and peaceful until a pair starts breeding - then they will be aggressive but not to the extreme that some other aggressive breeders can be.
I started with about 7 fry that I got for free at my local PetSmart, and now I think I have around 15 (more breeding).
They're very easy to breed, and they spawn in caves with eggs and guard the fry. This makes them a lot easier to deal with than mouth brooders which you might have to strip (and this can be difficult).
And stores will take them for store credit usually. Unlike those poor convicts.