I've successfully kept angels and gouramis together. I don't know why you couldn't do it... I think that gouramis and angels are pretty equally nippy.
You would only want 1 gourami per tank, unless you could definately get girls. Not sure how many angels in a 55, but I think it is something like 10 gallons per angel, so that would make 5. I think that you get problems if some breed, but I don't really know.