A pair of convicts will be fine in a 30g tank, however even with dither fish they can be aggressive when spawning. Convicts work very well with Oscars in a large tank as they have the extra room to claim territory unused by the Oscar, in a 30g tank a pair of convicts will likely be tank bosses.
If keeping a firemouth alone as well, it can quite easily take over a tank as even by cichlid standards, it can still turn aggressive on the flick of a switch. They are very mild mannered though compared to some Rift Lake cichlids and if given the right environment will color up nicely and spawn easily.
My personal experience having kept and breed a large number of various cichlids, is that they should be kept really in a 55g tank or larger. The extra room of the larger tanks, allows specific territories to be established and keep each species happy, with smaller tanks, there is less "real estate" for them and this can not only lead to aggression and constant chases but also to water parameters issues quickly. Cichlids in general by their nature are messy, big eaters and as such produce a lot of waste, not only poop, but also urine which can lead to ammonia / nitrite spikes in smaller tanks.
Am not saying keeping these fish is a NO NO, just that it will require work and you will really have to keep on top of water parameters. If you cannot provide this for them, then a species more appropriate for your tank should be considered.