I would:
First: get the betta his own little tank/bowl. 2 gallons or so. Or if you have 2 bettas then 1 small tank/bowl each.
Second: Try to find someone to take the bala sharks off your hands; they are going to need a bigger tank than your 55.
Third: Get a nice school of giant danios, say 7 or 8. That should make for a great-looking and very entertaining tank.
Fourth: Keep the plec, at least for the time being.
Fifth: Depending on the size of the plec, think about something for the mid-levels of your tank, cant think of anything at the moment. Or alternatively, a small school of corys might be able to share with the plec.
I don't think I would put slow-moving fish like gouramis in what's going to be a fairly lively tank (with the giant danios). This might actually be a tank that could cope with a ruby shark, as the other inhabitants are going to be fairly robust. Or is this a bad idea, folks (I've never kept one)?
Whatever you do, stock slowly!