Well....a 125 should work out fairly well. Do rock formations at both ends with an open zone in the middle.....I would get them in the 125 as soon as possible so they can split off and make some pecking order now. The males will develop the huge frontal lobe, and the dorsal, anal, and tail fins will be far more pointed and longer then that of the females more rounded short fins.
They will start pairing off at a young age so the quicker you can get the 125 set up for them the better. Make sure your base rocks are completely on the bottom of the tank for they will move some sand/gravel around when they nest.
Not sure how many you have, but a good round number would be 6 total and pull any that get beat up.
They do not have to be full grown to breed....however the spawns will be far more stable once they are.