20ss are a standard size. Creative crystals in Florida (www.creativecrystals.com, I think), is a good place to order from. Those 10ss are harder to work with--so small, and I found they fell out of the tweezers.
I've never stoned a costume from the beginning; I've only replaced stones. Recently, though, I had to add hundreds of stones to a costume. My teacher suggested I use hot glue--that the stones would stick better. Instead, I used Gem-Tac. (I haven't done heat-setting.) I didn't get glue on the costume, though. I held the stones in tweezers, touched the stone to the glue, then the stone to the costume.
Also, go to www.danceforums.com (google "dance forums", and the correct site will come up if I've left out a hyphen or something). There are many threads on that site--use the search feature--about stoning costumes.
Small stones WILL show up, though. Especially if you're using a stone with aurora borealis in it--those look awesome on the floor!
Good luck! Also, look at the pictures of costumes on rhythmicrentals.com--there are some close-ups of stoning.