Well for one thing, don't get 2!!! But other that that, Beta fish are pretty calm around other fish.... let the bag that you get the fish in sit on top of the water in your tank for a few minutes, i suggest 10 - 12, after the first few days, the beta will be territorial and may nip, so watch out.... if it gets to aggressive after the first week, you may have to but into just a glass fish bowl.
The other fish are even more likely to attack your new Betta, than your Betta attack your fish, so be aware of nipping of the tail
Definitely get a female, they are less aggressive and more calm
And also, remember that it all depends on the fish, you may happen to just have bought a bad one
Good Luck!