Even one convict is way too much for that tank. They get 6 inches long at maturity, and pretty aggressive. They will start killing each other very soon, or your water will be so bad that it will kill all of them.
And a lot of people get told that they need fish to cycle a tank, but you don't. You can do a fishless cycle by just setting up the tank and 'feeding' it with fish food as though there were fish in it. You can also drop a little bit of raw fish from the grocery store in there (or shrimp, I guess). Cycling with fish is pretty cruel to the fish; you are giving them a poison bath that will at best damage them, but probably end up killing them.
I would recommend taking the convicts back where you got them as soon as you can, and get something more suitable for that size tank. (once it is cycled, of course!)
No, I did not keep the convicts. I just borrowed them for the first 4 weeks to help cycle the tank, of course with constant monitor of the water parameter. They were small and seemed ok, and still thriving in my cousin's tanks now.