55, the jd will grow large and will need a 55 by himself, youd be able to squeeze the firemouth in there but watch for aggression. And yes keeping a poly with them should be fine.
55 are good for alot of the female counterparts such as jd,tex,gt,rd but males almost always outgrow the 12in width. I have a female rd in a 55 forever now and she is about 9 1/2 in. female jd with firemouth is your best bet.
Go with a 75, you and your fish will be happier. It's only 20 more gallons but that 6" of width makes a big difference. Especially since you want to keep a poly. I'd recommend a Delhezi for a smaller tank like that.
I have been told a 40g long would work with something like you are questioning. I have never had one before. Just word of mouth. Always try to check the dimensions of an aquarium. Long and wide wins.