Hi Rebecca,
Keeping a 10 gallon saltwater tank can be done -- but you're going to have to keep a very close eye on your water quality to make sure that nothing gets out of whack... which can happen very quickly in a small tank. In general, I'd recommend starting with a larger tank... but if you're willing to put the work into a small tank, it can be done.
For fish, you really *should* just stick to one fish... you could probably get away with 2, as long as they're small. Either one of the smaller clownfish (perculas are my favourites
) or a damsel should be ok... and for a second fish if you want to have 2, maybe a small goby or blenny.
If you plan on having corals, you definitely need to upgrade your lighting. Depending on what you want to keep, that can cost quite a bit. For the easier corals (most soft corals for example) you should be ok with 2 flourescent bulbs -- one full spectrum, and one actinic. Mushrooms and leathers are probably your best bet. They're relatively easy to keep, and don't require as much light as a lot of other corals. You could probably also put some polyps in there (button, star, clove...). Any of the hard corals require more lighting than you're probably willing to invest in (power compacts, or metal halides) -- same with anemones, they have very high light requirements.
For inverts, you'll probably want some hermit crabs and snails to act as a cleanup crew... they'll eat any leftover food the fish don't get to, and also clean up algae. Feather dusters are really neat, and should be fine in a tank that size. You could get a small serpent or brittle star, although it will probably get too big for the tank eventually. There's also several kinds of shrimp that look great in the tank -- cleaner shrimp, coral banded shrimp, fire shrimp... all cool
I wouldn't recommend putting seahorses in the tank unless you plan on dedicating the whole tank to them -- they tend not to do well with other fish in the tank, since they're slow feeders. If you're willing to put the effort into their care though, they're simply amazing
Hope that helped a bit, if you have any other questions feel free to ask away
I'm in the process of setting up a new saltwater tank myself after my move... I miss the tanks that I had out there, unfortunately I wasn't able to bring them with me so I'm starting from scratch again....
Jen