I admit I didn't read through every post thoroughly, but if the goldfish is anything but a common....The mods on my goldfish forum said that a 40 gallon breeder tank is suitable for any single goldfish of any type, and goldfish do not require the company of other goldfish, although they can provide stimulation for each other. So if you absolutely can't get a 55 gallon you can get a 40 gallon, probably a breeder(long).
That is just what the mods on the goldfish forum have told me. Hope this helped...
5G - 1 betta: orange dalmatian VT male "Ty"
29G - 1 orange fantail goldfish "Goldeen"