I suggest you read the thread below started by Jon@ Work - there is alot of info there.
Male bettas need to be kept by themselves - you cannot keep them together. In my opinion minimum tank size would be 2.5 gallons. Females are often kept together in what is called a "sorority", but they are perfectly happy being by themselves as well. DO a search on female betta sorority for more info on that. I'd suggest starting with a single betta to get the feel of things.
The best type of betta is the one you can afford - if you can afford to have one shipped from a breeder then go for it, if not find a healthy one locally. Both will be rewarding pets.