CTLove: no, that sounds like the kind of aggression that I would break up. You can separate them for awhile(more than a day in my opinion) and then try to re- introduce them to sorority...sometimes this works. If you know one of them is the aggressor, you can just remove that one, if unsure, take both out. On another thread, I believe I saw the recommendation to remove all sorority females and then re-introduce the whole sorority to tank at same time....which is a good suggestion, but in my opinion, if tank is heavily planted, it is not necessary.
I have two sororities and I frequently remove a female for breeding purposes and then re-introduce later with rarely any pecking order issues, but: 1, my tanks are heavily planted with many hiding places and line of sight breaks, and 2, each sorority has over 5 females. It is ESSENTIAL to never put less than 3-4 females together...they need to spread out aggression and territories or, more than likely, you will have a fight to the death!
+1 to babystarz...follow her advice and you should do fine!....looks like everything covered well.
LRucker: the heavily planted 10g you described should work great, and rescuing a bunch from Petco will be fine as they are usually young fish. On side note: we have lot in common as my 30+ betta obsession began when I bought a 2.5g tank and male betta for my 5 year old daughter! And your right: compared to most fish, bettas are bursting with personality. I speak to them like I do my dogs, they have to be named, they respond to world outside tank...