Well, back in the olden days, we kept a pair of blue acaras & a pair of firemouths in a 55g. It wasn't great, the acaras took up almost 2/3s of the tank but the firemouths were able to breed too.
Convicts breed like crazy!!! & for their small size an amazing amount!...whee, & they're very agressive then too..Ugh. I would not suggest convicts except in a species only tank...& really not then either...Think of 100+ fry every single month...yeah, it's not very fun after the first time...what will you do with all those fry? There's no market for them...you & they will have flooded your nearby lfss after the first batch or 2...
Sorry to be a buzz kill but you need a better plan than any I've seen with your proposed fish...it's just not a good idea even in the very short term.
You might think about apistogramma dwarf cichlids. Do some research...you might keep 2 pairs in a 55g...but then you'd need a 20 long+ to raise them to a sellable size...for each species....Do more web searching & hard thinking...many cichlids are not very hard to breed but they can be difficult sell or even rehome...Can you sell in a big market area? Selling by mail can have issues too...think hard!! Everyone can breed convicts...