Ive heard seahorses are hard to do.. I'd love some some day myself.
Damsels, Clownfish, grammas a few good 'starter' fish. Common clowns are pretty good community fish.. I'd steer clear of damsels if you want to have more fish.. and I'd go with a pair of clowns for a 55 gallon. Thats what I stated out with.. but its up to you. Fosters & Smith has a good 'starter fish' section on their liveaquaria page. That cardinal fish you mentioned would be a good choice, too.
I would get a good skimmer - if you're not wanting to go the sump route you can get a decent HOB
skimmer if you're going FOWLR
( fish with live rock, or fish only) - ideally having live rock helps a lot. 1-2 lbs per gallon..