yep, you can add more than three they don't really get huge but if you plan on them breeding it's a good idea to have some space. having three or four you could be able to get a male and three females and you'd have a good chance of having some fry that don't get eaten.
There is plenty of room for the three swordtails in there. They don't get extremely large and that tank size should provide plenty of moving space for them. You might want to look into Corydora Catfish. There are a lot of interesting color combinations for them, and they will swim on the bottom of the tank, giving you different levels in your tank.
Well, you shouldn't keep cories with mollies because mollies need salt and cories don't like salt added to the water. Otos would be good in the tank though. You shouldn't fill yoour tank though, mollies will breed quickly and you'll hae lots of them.
Oh, I totally misread that, but your Mollies will be fine as well. The sizes are about the same. I would go with all females if you can, as breeding sounds interesting, but really ends up being a hassle later unless you're well equipped for it.
Mollies don't need salt if water conditions are kept very clean. I should have mentioned that. I overfilter and understock my tank with Mollies in freshwater and I have had no problems.