If you decide to breed firemouths, I would suggest leaving the two tanks for only firemouths. You can have one tank at first, but your fry will eventually grow bigger. How many fry you have depends on different factors such as age of your breeding pair, but even one spawn will yield you more than enough fry to fill up a 30g when they're 0.75-1" each. If you're looking to breed for profit, you'll actually need three tanks. One for the breeding parents, one for the fry you raise, and one for "ready to sell."
If you plan on getting rid of most of your fry, you could have one breeding pair in each of your tank. Since you want to have a pair of fish in a 30g, and also have space for newly born fry, you can't keep most SA/CA cichlids because of their maximum size. I would suggest some dwarf cichlids, such as blue rams. They're not too difficult to breed either(but not as easy as livebearers and convicts).
If you want to breed for money, you'll have to breed a fish that is high in demand. If the fish is too easy to breed and everyone can do it, then you won't earn much money from the breeding. Also, you might not be able to sell all of the fry. Your initial starting cost would be higher because you would have to pay for more expensive fish. Discus for example are high in demand, and you could fit a pair of discus in a 30g(10g per adult). A proven pair would easily cost you $200+ though, and breeding isn't guaranteed.
Good luck on whatever you decide to breed
