can I have a saltwater 10g?

Absolutely no reason why not, but you'll just have to remember water params can change quickly in such a small tank (well, just as quickly as it does in freshwater, but marine animals have adapted to little to no changes in thier enviroment, so they aren't the most tolerant of this), so I would wait until you have confirmed that the water stays stable before you start adding fish/coral. You should wait atleast 3 months before adding soft corals in my opinion, and 8-12 months (depending on how well the water is going) before adding hard corals (LPS and SPS) and/or anemones.

A single clownfish will have no problems in a 10g, but I would try to get either tank bred or tank raised clowns, as these are hardier, and I would also try to get percula or occeleris clowns, as these stay small (about 3 inches max., small for marine fish).

For equipment, I would try a HOT seaclone skimmer(buy the smallest one possible), and about 100-200 gph of powerheads in the tank. Lighting depends on what type of corals you wish to keep, but if you want an anemone, I would go for 96w of PC lighting and get a bubble tip anemone (hardiest of the anemones, but still hard to keep nonetheless). Add to that sand (preferably aragonite, but pool filter sand works in my expierience) and a mix of live rock and base rock(can be all live or almost all base), about 10-20 pounds of it, and your good to go.

Please, do not jump into this right away, you need to do extensive research on the tank and everything that goes in it, and have a flexible plan for it as well (eg. for stocking list have: 1 percula or ocelleris clown, 5 blue leg or scarlet hermits, 10 astrea or nassisarus snails, ect...). I suggest you read all marine articles we have herehere[/URL] at AC, and read as much about marine tanks as you can in the following links:

Age of Aquariums

Aquartices

Dr.jungles World of Marine Aquariums (see reef aquarium as well)

WetWebMedia

LiveAquaria (look under resources for articles, should be labeled "Information library")

I'm guessing it's becuase they have adapted to the stable water conditions better than others (less tolerant of changes in water params), and becuase they tend to die very slowly, so it will be hard to identify that the water or something else is not to thier liking, But I really am not sure.

Please note: I had to add some things to my original post.

Sounds good to me. You could add a second clownfish, and few algae eating crabs/snails (only add these if you get a small algae probalem, and research what will eat that specific type of algae), but always remember less is more.

Yes, instead of a second clown. You could add pretty much any fish as long as it stays under 2-3" and is compatible (behavior wise) with the other planned inhabitants.