It's never a good idea to bank on a fish become stunted in an aquarium of a certain size. To the best of my knowledge, it only really happens with some schooling cyprinids (e.g., goldfish). Obvious examples of fish that get big regardless of the tank are oscars and plecs, which frequently outgrow their aquaria.
A 55 gallon tank is a fairly decent size, but I don't know enough about knifefish to comment on whether it is ideal for that sort of fish.
Spiny eels should be okay, provided you keep the smaller species. There are at least three species sold as peacock eels, and the hobby literature is hopelessly confused when it comes to applying a Latin name to a given fish. I'd encourage you to check out the Fishbase entries on Macrognathus aculeatus, Macrognathus pancalus, and Macrognathus siamensis before spending any money. The smallest species, Macrognathus pancalus, tops out at 15 cm, while Macrognathus siamensis can reach almost three times that. Macrognathus siamensis is probably the most frequently traded (in my experience).
Spiny eels will work fine with angelfish. Contrary to popular belief, these are not brackish water fish, though some at least are salt-tolerant. Neutral, soft to moderately hard water suits the Asian species best, though some African species prefer hard, alkaline water. Spiny eels are predatory but quickly adapt to frozen foods, but at least to begin with you may need to provide live Tubifex and earthworms.
All spiny eels are extremely prone to bacterial infections of the skin. Improper substrates seem to be the prime cause, scratching away the mucous layer allowing opportunistic bacteria to infect any slight scratches that would otherwise be harmless. In my opinion, you should always keep spiny eels in tanks with sand, never gravel.
Cheers,
Neale
