The rule of thumb generally is 1 inch of a fish per Gallon, most female betta are 2-2.5 inches, but they can be more and less, i have a female larger than most of my males, shes a bit more than 3, also some one else on here has a HUGE female. Also the more female's in a crowded space will make more aggression and because there is less room for them to hide. If you are going to start a Sorority Tank (female betta tank) I suggest many decorations, dense plants, and lots of hidey holes! A 10 gallon is a perfect size for a starter tank i think. Its what i started with and still have and love! At my tanks peak i kept 6 females , i wouldnt go more than that though. What i have learned is you really have to watch them for a few weeks, and honestly i think they can go hostile at any given time if they get aggrivated or threatened. From my expericance no type or size has a different. My larger female got along with my tiny females just fine, darker with lighter, crown with veil tails. I would make sure you are seeing each fish eat. If you have a few that wont eat, i would suggest putting them in a bowl or cup for a while and teaching them that the pellet and you and your fingers are good!!! they bring food! Then re-releasing them. Also Ghost shrimp add some fun flair to a tank. My girls love inspecting them. I had a few chomped. But they are cheap! 10 for $1.00 around here... so thats awesome. Also if you have moss or marimo balls in your tank they can keep them clean looking with no poo or debris on them. Here is a Shot of tank as of a month ago, it is now a little more dense, the wisteria in the back has grown, i have sprouted a new bulb, and took out that fake looking one middle right, and it has these cool lily pads at the top all the way from the bottom and the red plant in the back left has really flourished! I plant on adding some kind of fore ground plant, prob Echinodorus tenellus.
Welcome to the forum! As it was said above, males and females can't live together. You can divide the tank with dividers and put a male and female in a tank, but they can't roam around freely unless you wanted to breed. And Alex explained everything about the female sorority.