The only real concern I'd have with a 29 gallon tank is that in close quarters, a lot of New World cichlids have a tendency to turn on one another if something goes wrong. I've had several species, from severums to Laetacara to angelfish, have disagreements about raising fry and then one of the fish (usually the male) tries to murder the other one. A tank that size, with two 4 - 6" fish, doesn't give them much room to escape one another. You'd want to really keep an eye on them.
If you could find a 40 breeder that's probably a better foot print and you could leave the fry with the parents a bit longer which would likely curb their desire to spawn again.
IMO you need a larger tank than 10 or 20 gallons to grow out fry to salable size. Most LFS won't take them until they're 1.5 - 2" at least, and when dealing with 100 or so fry (usually it's more than that but let's just make it a nice even number) that's never going to work in a tank that size. Even 50 fry at 1" is too much for a 10 gallon. When I raise out fry, I tend to grow them out in 55 gallon tanks with large daily water changes. Plan ahead before you start breeding because you'll be growing these fish out for several weeks and if the tank is too small/conditions are poor due to dirty water, you run the risk of bad fin development and poor growth. You won't want to put in all that work for nothing.