Agree with this.
FMs prefer to live in a group of at least 4 or 5. of their own kind
A 40 gal tank is not adequate for either scenario
If alone, or in a pair, they tend to be hyper aggressive.
Whereas GTs on the other hand, are loners, but at that size, if the FM may overwhelm.
It also depends on tank size, if they perceive the tank as not enough space, (and its up to the fish to decide how much space is enough) not you.
A group of FMs with tetras as dithers, need a minimum 6 ft tank (you also need at least 6 or more tetras to be effective as dithers), hence the need for a 6 ft tank.
because your tank is smaller, you'll need to get rid of 1 of the cichlids,
and in reality a 40 gal is not enough space for either cichlid species alone as adults..
