That's not a mix I'd ever suggest. Balas are prone to stress, and convicts are extremely aggressive and territorial. In a tank that size, 2 convicts (especially a male/female pair) will claim the entire tank for their own very quickly. If the 2 convicts pair up and begin spawning, a tank that size will prove challenging to keep clean even for the cichlids, unless other arrangements are made soon after the fry arrive. Convicts are good parents and breed easily, meaning that any other fish that comes near fry will surely be attacked. The joke I used to tell about them at the store to get my point across is that they didn't get the name of "convict" for no reason.
If, however, the tank were over 100 gallons with 2 balas and 2 convicts, with enough decoration, there's about a 50/50 chance of pulling it off without the cichlids taking out the balas.
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