Juveniles, no problem. Once they start maturing a pair will form and the odd ones out will be at their mercy. That pair will claim the entire 30g, and should be fine as the only inhabitants.
I tried a firemouth+convict a year ago in a 20g long, and the convict grew faster than the firemouth and got too aggressive. Small tank, nowhere for the firemouth to hide so he was stressed. I rehomed the convict, and since then upgraded the tank. I now have a 38g, with lots of caves and line of sight breaks, with the same male firemouth (bigger now obviously) and another smaller female convict. They tolerate each other.