How agressive are you going for? How about:
1) Wimple piranhas
OR
2) Exodon Paradoxus (Bucktoothed tetra)
You could do a school of the exos, they wont get bigger than 7". You could also go for the good old cichlids too; If you do that, maybe the malawi eye-biter? They are quite agressive, and not over 10". If it is just one fish by itself, I would go with the eyebiter. It is a truly nasty fish. Would you be willing to consider eels?