30 gallons would be a minimum size for a breeding pair of Convict cichlids too. Course, you'd have to figure out what to do with the fry.
With 30 gallons, you could probably do the ideal platy/variatus set up - 10 females, 3 males. Again, you have a fry issue.
One male, three female swordtails and some firemouths would be cool (I think the firemounths would solve your fry problem).
Tanganyika shellies have been mentioned, but you could do a pair of Julis in there, or Brichardis. I don't think you would have a problem unloading Juli or Brichardi fry.
I have a couple other ideas, but I need to work them out first.