Yes, its fine to have an airstone in a saltwater tank. Some of the fish enjoy swimming through the bubbles!
If you have no fish in your tank now-FIRST
make a list of your favorite fish, most favorite at the top, then going down!
Then check to see if they are compatible with each other.
Then buy ONE, the least aggressive should be the first fish in-the most aggressive should be the last one added. In between additions, allow time for your tank to adjust to the increased bio-load, test for ammonia and nitrites. Wait about 10 days to 2 weeks before you add your second fish, making sure there is no ammo or NITrites.
If this is to be a fish only tank/no corals, then you can carry a larger fish load-again dependent upon your filtration and commitment to maintenance.
In a 50 gallon reef tank, I would only suggest 3 smaller fish.
In a fish only with live rock and good filtration, you could have 5 smaller fish, several shrimp, serpent and/or brittle star-the things that make a saltwater tank really interesting.