The only time you shouldn't move an FM back to Rome is when they are "Prospective Truimphator" you have to wait until the Senate make him a "Truimphator" before he can enter the city, usually takes a couple of turns.

You will often get "Imperator" fighting rebels but you will never get a Truimph out of it you have to fight factions for that, and that is determined by how many cities taken battles won and how hard the odds were!