I have my FG in an 80 gallon and he's about 8" long. He has lots of room in there so I'd imagine you'd be ok in that 55 gallon for a little while anyways.
i would not buy a FG and keep him in a 55 gallon for long. I bought mine at 7" two years ago and he was ok in the 55 gallon for a month then i put him right away into the 150 gallon.
Its not suggested because you can stunt the growth of the gar. And they do grow fairly fast to about the 12" mark.
