Here's what we did, it's really not as hard as everyone makes it out to be (in our case.)
Our betta girls weren't sisters, they weren't even from the same store. We went around until we got 6, got our net ready, and then quickly dumped them in. Luckily, none of ours were that aggressive, but thief was some fighting. We have a 10 gallon. Correct me if I'm wrong but you need quite a lot for a large tank. Also, ours really don't use the hiding spots that much. We went to our local michaels and picked up the fallen fake plants of the floor, washed them, and dropped them in, they luv it. Good luck, and be sure to have a net!