My sister has neons in with her betta, and he killed them all. She has some rummy nose tetras in with him now, and even then he can be quite aggressive to them (he took one of their eyes out).
I was wondering the same, but since then I've found out that overstocking your tank over the limits just isn't a good idea. I had 3 platies sharing the tank with Victor, but I sent them to my sister's house because I realised it wasn't a good idea to house them all in such cramped space.
I was considering getting some neons too, until I heard that they don't really get along with bettas that well, and the fact that my tank just isn't big enough. The people at the pet store should really know better.