My DG harrasses my P. scalare no end. I don't know why the Angel tolerates it, but it happens. No biting, generally, just chasing.
Summarizing what, from above sounds correct to me...I would suggest, at most, 2 pr for a 55, making sure that you have a nice, vertical piece of slate or other appropriate rock at each end of the tank. Med/larger tetras are OK (neons, sometimes even cardinals are occasionally eaten by Angels). I'd avoid Gouramis or have just 1 dwarf if you must have a Gourami.
I would also suggest having several visual breaks between ends of the tank (tall plants, structure etc.).
PS. - if you go w/ Altums instead of P. scalare, or if you go wild caught instead of tank raised, all bets are off, count on modest success w/ just 1 pair if and only if you keep the water super clean and "Amazon-like" (I think that means fairly soft, somewhat low pH - I could be wrong on that).