Are your convicts full grown? I've kept firemouths with convicts with pretty good success. Maybe a small terror of some sort, you could even try more convicts. Thats what i got now, a swarm of 7 convicts, who have killed everything that they come in contact with.
What size tank are they in now, cause tank size will have a lot to do with how they act. There needs to be space for everyone. Also what fish have your cons killed so far?