There are cases were they co-exist with ocellaris clowns, but that is only in really large tanks.
There are a few species that can be kept in a 10 gallon, but they are very difficult
1)H. zosterea- the dwarf seahorse. You will have to hatch BBS daily, and feed it 1-3x a day
2)H. capensis-an endangered species that was thankfully bred in captivity, before they became endangered, so they are fairly common in the aquarium trade. They eat frozen mysis, but they are a coldwater species, so you will need a chiller, unless you can keep the tank between 66-70 (70 being the hottest I think you should go) at all times.
3) some other assorted species, that either need BS, or a chiller, and are failry hard to locate.
I would also like to say that I agree with joker, that crabs are a bad idea, except for porcelain crabs (which technically aren't true crabs....).
Joker what kinda bad luck have you had?