I have done the same thing in a 55 and failed. They were introduced at the same time convict being 4", FM 3" and GT 2". After a couple of days GT gained the upper hand over FM despite the size difference. The GT quickly outsized them in 2-3 months and then I added an 6" synspilum. The bigger synspilum didnt let the GT dominate and grow for 3 months till I get rid of it. In this period the FM constantly picked on the GT, taking advantage of having a big brother (synspilum). I think he knew the GT would be a danger for him. After the removal of synspilum, GT again started growing and the FM tried to set up alliances with the hardier convict but the convict didnt seem to like his company much. The GT was harrassing the FM and one night boom! FM was dead. Unfortunately I wasnt there for weeks, if I was I would have removed the FM weeks before it died. Now the GT and convict are the only occupants, but since I made a cave for convict I feel safe about it. So no to a male GT with a FM in a 55.