Give it a shot. You're right he is not very well matched in the other setup. He is a CA fish so would do better in CA water conditions than with the SA fish, but if it's an aquarium strain FM I'm sure it's adaptable. I have kept Steindachneri and did not find them all that aggressive, but they do have the same feeding behavior as FM so maybe he will see it as competition. Can give it a shot, and if it doesn't work out, perhaps re-home the FM. They do much better in groups, seems like neither setup is ideal.
Agree, there are reasons why there is only one Geo in the local area these Geo's come from, and because a FM is the Central American equivalent to Geophagus, the similar looks and territorial needs may be the cause of instinctual friction.
