How about a chanchito? I have a couple dozen ~1.5" juvies available, could ship you one Priority Mail for $20. The proper name is Australoheros oblongum. The males grow to about 5", females a little over 3". About the same size as convicts, but much better disposition in general. Mix pretty well with anything they can't eat IME, and thrive in hard, alkaline water. Here's a few pics of the parents.
Dad first:
The little wife:
The happy couple together:
The little hooligans:
I do have a few more pics of the juvies, but photobucket isn't cooperating for some reason.