I believe BGKs get about 16-20 inches long, so a 55g wouldn't be big enough for a permanent home. I'd just get one, but plan on upgrading within a year or so (or whenever it gets around 12 inches).
The bichirs will also hit about 12 inches, so I'd plan on a bigger tank for them in the future also.
The ABFs should be fine. Just don't keep any small tetras with any of these fish.