How about 6 to 8 Columbian Red and Blue tetras?
I have 8 in my 75 community at work with a breeding pair of Angels and a flock of pristella tetras, 8 von rio tetras and some assorted inverts, cories, ottos, and a bristlenose pleco.
They don't have anything on exodons or even congos... but they're fast aggro little tetras with a lot of speed and watching them dash at the feeding ring is reminicent of watching baby piranhas.
Your convicts might take them out, but I don't think so.