Bettas are raised in cups and do not have contact with other fish unless you happen on a very good pet store that puts them in the tanks. They are, as a result, not sure at all that other fish have the right to occupy the same water that they are in. They are territorial and skittish with most fish and it seems that most of the fish that they do not bother think the betta's fins are open for nipping or chewing on. They need to have fish around them who are going to stay out of their way and not put themselves in harm's way.
Rainbowfish are quite a bit bigger than bettas and need to school so you would need several of them. It would be intimidating to a fish who has not been about others to be in a tank with fish (and a school of them) 1 1/2 to 2 times his size. He would more than likely be aggessive to bluff and blow or else depressed and inactive. (Yes bettas do get depressed)
Tank mates known to get along with bettas are Corydorascatfish or if the tank is smaller or you want a few Otocinclus catfish. I am going to put a betta into my tank with the Dwarf Loaches but it is going to be an experiment and I have another tank for him to go to if it does not work. The thing with putting bettas into a community or mixed species tank is every betta is different. you may get a lamb and he will accept almost any other tank mates or you may get the little demon who will kill everything you put into the tank and then again you may get the little guy who is so laid back that he lets the others nibble his fins to bits and ends up dying of a horrible infection in his fins. Bettas have such distinct personalities that you never know what you are getting until you get them home and into the tank. ALWAYS have a back-up plan and another container to use if the situation warrants it. Please never just put a betta in with other fish and walk away. They need supervision for a period of time and that will only be with luck that nothing happens one way or the other during the night while you are sleeping.
It's hard to say. According to the compatable fish chart you would have to use caution.
What size of a tank do you have? I would say that the larger the tank you have, the greater your chances are for success. Also you would need to have lots of places for the betta, and/or rainbows to hide in so they can feel secure.
I am assuming that you have a cycled and heated aquarium?
As Rose said, you would need to have a backup plan in case there were problems.