FM would be fine in a 55 gallon. You could do a pair and still have tankmates.
With the correct decoration you could have 2 pairs in a 55G, but I wouldnt really recommend tankmates.
If you want 1 FM:
1 FM
6 swordtails (2m 4f)
And you would have room for another cichlid/more dithers if you want eg. a convict, rainbow cichlid, theres plenty of options. All up to what you like, research some fish then ask when you find some you like.
If you wanted a pair of FM then I would do:
Pair of FM
6 swordtails (2m 4f)
If you have clear territories and decorations to break up the line of sight, you MAY be able to house 2 pairs of FM, but it will be risky.