It def depends on the temperament, i have a FM that lives with 3 rainbows, 2 dwarf rainbows, a Guamri, oh yeah and a smaller Bolivian ram, strangely enough my FM is by no means aggressive more of a passive, as soon as he goes NR the Bolivians space the little guy will come charging at the FM and keep butting him, normally the FM just turns his side and lets the Bolivian take a few shots with the look of "come on, what you trying to do" only after a few (6/7) butts from the Bolivian my FM will turn head on with the Bolivian and then go head to head with him once hes done that once with both of them getting right up to each other but not doing any damage at all the Bolivian will turn round and go back to his corner and the Fm will carry on meandering round the tank. Ive never seen him be aggressive to any fish i have.