I would say probably not. Things get pretty tense when convicts are breeding, which is almost always. You would likely see the cons claiming 80% of the tank and everyone else cowering in the corner. In a larger tank where the cons can claim thier own protected corner you can get away with this, but I would be leary to try this in a 29 unless, of course, you used a divider.
I would say that as a general rule it is best to pick the fish and then go with a tank that is best fit for them through the long run. A pair of cons can work with say the firemouth that you mentioned but not in that size of a tank. If you are set on the 29, I would say the cons (and the cons alone) would work fine in that tank. If you really want that firemouth then I would say more like a 55-75 gallon.
I think that too often we cichlid hobbyists (me included) get a tank and then want to get as many fish in it that will survive. But if we stopped to think about the long term and what is best for the fish we would see that less fish may lead to more enjoyment because things in the tank will go much smoother.
But back to your question, and away from my rant, I think that if you go with the pair of cons in a 29 you will have two very happy fish and get years of enjoyment (and millions of little ones) out of them.
You can try... Convict fry are pretty common, so you certainly aren't going to make any money on them, if the lfs will take them at all. I think most people tend to use the fry as feeders for their larger fish.
most lfs won't take them because everybody's convicts won't stop breeding and everyone tries to sell them. You would be much better off feeding them to something, a few thousand baby convicts every 2 weeks can get a little crazy.
Depending on the parents, quite often if you just leave them in there when the parents want to spawn again they will eat the leftovers. If they don't then the 1st batch will eat the 2nd batch. If they start to get too big and the 29 gets crowded then use them for feeders or advertise them on craigslist as feeders.
I would say that the first step is to ask the LFS where you get yours from if they would take them. If they don't want to pay you they might just take them for free. Back when I had my pair I found a place that gave me store credit for some of the bigger ones I let grow out. If anything they might use them as feeders for their own fish.
But as far as people that will want their own convicts from you that will run out fast. When mine spawned I was able to find a couple of them good homes but after a while I ran out of people that wanted them as pets. ...I guess I don't have enough friends
I hope we didn't all turn you off the cons. IMO they are great fish and I loved my pair that I had. You will get tons of enjoyment out of watching them. I also think that they are great for someone wanting to get into breeding cichlids because there is tons of info out on them and they are very much do-it-themselves when it comes to breeding. Seriously I would go to a LFS that you are thinking about getting them from and just ask if them would take fry.
If we did talk you out of the cons then maybe looking into: Salvini, Sajica, Rainbows, and Firemouth. I think that all of this cichlids are good ones to start with. Some of them (the Salvini and Firemouth) would require a bigger tank in the long run, like a 40-55 would be best.
Once again, I really hope that I didn't scare you away from cons. I really think they are great fish that anyone would really enjoy. Let us know if you have anymore questions.