I think there's a few questions mixed up in there
You can keep females together no problem. As soon as you introduce a male to the mix, you'll likely have aggression problems.
Min tank space for each male betta is quite small, as you can probably see by how a lot of people keep them in half cut - coke bottles and jars or the way they're kept in animates before being sold. . Don't do this, it's not nice .
I've found all my bettas to be a lot happier when they have at least 20l each. By happier, I mean it shows in their behaviour. They are a lot more active and "inquisitive".
No need for a bubbler =)