Cool :) Do be careful with that many fish in a 6g.. a couple would fit, but 5 you generally need a few more gallons. The bio load would be big, but you can do extra water changes.. what you have to be mindful of is the space itself. Sometimes fish can feel "trapped" in smaller tanks, and will become stressful, die and/or become aggressive due to it. I have the bookshelf aquarium right next to me, and with the size an adult female betta gets to be (mine are larger than adult males who are older than them).. you would only want to put 3 at the very most in that size of a tank. The females do get large and are more mobile than males.. they aren't territorial and don't set up territories such as a male.. they are nomads and wander around. So placing that many in a small confined space you are asking for trouble.
Not trying to discourage, but if you want 5 girls you will need to get at least a 10g.. and I wouldn't place more than 5-6 girls in that even.
So space is going to be the issue with you.. the girls will be small when you get them, but they will grow big, and they will kill off a few girls so they don't feel cramped/trapped.
This was my bookshelf aquarium divided 3 ways (no longer divided x3).. as you can see, it's just enough room for them.. so having 5 girls loose together in the tight quarters (it's rather short front-back) is not a great idea.