In my experience with convicts I've learnt the following:
1. They shouldn't be kept with shy, easily targeted fish like elephant noses, etc.
2. When you have one female they are "usually" fine with other robust cichlids.
3. If you have a lone male they are "occassionally" fine with other robust cichlids (depends on if he decides to start nesting on the offchance a female may just happen by
).
4. If you have a pair then "most likely" everything and anything else kept in the tank is gunna get it's butt kicked sooner or later or harrassed to the point of death by stress. It doesn't help when the male and female will often gang up on the offending tankmate and proceed to tagteam it into submission.
5. If you ask an lfs if they will take the fry (trust me, if you have a breeding pair you are gunna get fry and a hell of alot of them very, very soon. They are best mates with storks it seems...) then they are almost guarenteed to tell you to bugger off.
6. This is the most important thing I have learnt - a breeding pair of convicts in a tank means you should expect to never, ever do any work in or even near the tank ever again while they are residing there. They seem to have a blatant, uncanny disregard to the fact that fish need water to live. This behaviour is often exhibited whenever you pull your hand well out of the tank after being latched onto only to realise that the damn fish is still there or when innocently walking by, you are suddenly assulted by an overzealous fish who thinks that walking within two feet of the tank constitutes the opening of hunting season and you are more than fair game.
In short, they are a nice, interesting fish that can turn into a right mongrel when they want to (and in 99% of cases they will)