In a 55g, I would go with 5 - 6 juvies & let them pair off like minnnt suggested. It's a lot more inexpensive than trying to buy an already mated pair. A proven mated pair (that is a pair that has successfully spawned & reared fry can easily fetch + $100 USD. With 5 - 6 juvies you should get a pair (maybe even two) & you can re-home the others. Like others stated, generally speaking, breeding pairs of cichlids are intolerant of others, especially their own kind.
In a 55g you could house a pair of Angels with compatible tank mates like apistogrammas, rams, keyhole cichlids, cory cats, & tetras.
Personally, I'd go with a small school of cories (3 - 5), another pair of cichlids (either german blue rams or keyholes), & a school of 6 - 10 medium sized tetras (either black neons or lemon tetras). In a larger tank (90g+) you could use smaller tetras like cardinals, rummy nose or glowlights, but in a 55g it's not worth the risk IMHO.
Also keep in mind that if you ever decided to breed your Angels then you'd need to move then out of your 55g community tank into their own breeder in order to achieve a successful spawn.