Galaxy Rasboras are actually Celestial Pearl Danios. I owned them a few months ago, great little fish. They are quite small at about an inch long each, but a very shy and quiet fish. I would be concerned they would be scared of your Betta. Mine were terrified of my Gourami. They are fast swimmers though. I think they would do OK in a 10g tank. I had 6 and 4 Glowlight Tetras in my 10g. When I added the Gourami, they were upset so I found them a new home.
Mine did fabulous in 78F water, and our local breeder keeps them in tropical water. They are a new fish, and harder to find.
My 10g has 6 glofish and my male betta, Been that way since last August. Over thanksgiving break I came home to a glofish with 1 eye, she's still alive, so I think it is very doable. Mind you there isn't really enough room for them to school. When they do its magnificent but that is a rare sight, most of the time 3 of them will tag along at once but still fun to watch.
Well, after some more research, I've decided to try a sorority instead. I went out today to buy some silk plants to provide more cover in the tank. I intended to get more live plants as well, but couldn't find the ones I wanted. I have several small, colorful glass cups for hiding spots as well as a plastic reef. My tank is cycled. I have four girls so far. I plan to add them to the tank on Monday. In the mean time, I'm getting to know their personalities and enjoying it very much.
Personally, I'd hold off on adding them until you have at least 5. If you can, get all of your girls (up to 6) before adding them. Otherwise, each time you add a new one, you'll have to rearrange the whole tank and cup everybody.
But, yay! Sorority!
I had to do this with mine, kept changing the tank everytime I got a new girl. At the end of it, I bought my two new girls and tossed everyone into the 15g (was a 10g) haha When I do switch them to the 29g, I'm buying a ton of females :) Good luck!