This may be more trouble than it is worth, but if you are really attached to these fish and want to keep the four of them in the 75, here is an idea that might just be crazy enough to work. All you need is a big trap and a QT tank. If your gbta is on a small and/or movable rock this will be easy.
1. First make sure temp, PH and salinity are all the same on all three tanks, easy to do.
2. Remove the percs from the nano and move them to you QT tank, also easy.
3. Then sink a large fish trap into the 75 and carefully put the nem-rock into the fish trap, you will probably need a big one like one of Marc's big traps. The maroon will go into the trap easily to be with his beloved nem. Then gently move trap containing the maroon, the rock and the nem to the already established nano, and slowly sink it to the sand. You may not even have to remove the rock; just let the maroon continue to host the nem on its rock inside the trap, just tie the door open. Because the rock and nem will be the most familiar thing to the maroon he will probably feel comfortable right there inside the trap if it is big enough.
4. Add a new nem to the 75 on one far end of the tank, and give it some time to get settled in. Put the percs in and let them establish a new territory in the new nem in the new tank.
5. Then later re-introduce the maroon on the opposite end of the 75 by placing the nem and rock there.
Well. It looks good on paper, anyway. Probably what will actually happen is this:
The Clarki will take up residence in the new nem and not let the percs near it. Or. The two nems will leave their prospective rocks and meet in the middle of the tank. The nems will then join hands and skip off to some hidden cave and leave the clowns to duke it out.