Hello there!
I'm glad you're interested in buying bettas! However, there are few things you should know before you proceed.
As you already said, males cannot be kept together under any circumstances (well, that's something that can be discussed too, but let's leave it for the moment...). They have a tough nature of real fighters. That is
not limited only to males! Even females display such behaviors only somewhat milder.
Bettas do rarely form packs in nature, and if they do it's only a sorority pack. And it changes a lot, in a way that one day you have a group of 5 females and the next only one. Their hierarchy is very well known amongst them and they tend to show each other their place every day. Even a small change in aquarium can lead to a new "who's in charge" fight. If one of them dies or is being pulled out of the tank, the fight to take her place starts again.
That's not that bad as it may seem now. I've kept females together since forever, but you must be prepared to see some fights and if you think that one of them is a too big aggressor - to take her out!
That being said, once a hierarchy is estabilished, they can do a nice community that's not only beautiful to look at, but interesting also.
However, whatever fish-combination choose you will need to have a heater (if you're sure you're going for tropical fishes), a filter (in any case), and of course, a tank

The size depends on the fish you choose to have.
Best regards!
K.