Yes, I would think the blue one is most likely the more aggressive one. This is what I would do: Get 3 more females. Take your current two out and put them in cups or something else. Rearrange the plants. Now you have 5 in cups. Put the cups all in a row, so they can see each other and you can get a sense of their aggressiveness. Figure out what food everybody likes. Bettas are funny. Some like pellets, some like flakes, and I have one picky spoiled baby who pretty much won't eat anything except bloodworms. Whatever, just figure out what they will for sure be interested in. Then don't feed them for a day. Now that they are good and hungry, put the 3 new fish in the tank. Keep an eye on them for an hour or so. Flaring and chasing are normal. After about an hour, add your two current fish one at a time, keeping the blue one until last. Then feed them. Now you have successfully changed everything up so that there are no territories and distracted them a bit with food. Do all this at bedtime, so you can then turn out the lights after about another hour. Watch them a bit, but then send them the lights out signal, so they calm down.
They will chase, flare, and nip at each other establishing a pecking order. A few nipped fins are normal, just don't let anybody get shredded. If someone does get beat up, remove the victim for some r&r if necessary and make sure the aggressor doesn't start on someone else. If you get one that will not play nice, put her in your 55. You still have enough to establish a peaceful sorority in there and the one naughty one will be fine with your other fish.
As far as the divider goes, I had a pair in a 10 with a divider once. No matter what I did, the male was always ending up in with the female. Why he insisted upon doing this is beyond me, as she was not receptive to him and he would always end up with the beat up fins. I saw her bite him once. He drug her halfway across the tank before she let go! Anyway, after that he got the 10 and she went into the community tank. She is the one that killed the other female I added to the big tank. Who knows, she may have just been a royal ***** and may never have succeeded in a sorority, but she proved to me that 2 bettas is never a good number. Anyway, I gave up on the whole divider thing. I tried one to separate my male and female mollies in another tank. Didn't work either.
As far as long term behavior, what Paige says stands true. She has had quite a few females together before. Mine seem to have a revolving door of who is the toughest depending on personality and who is just feeling crankiest on any given day. They flare and chase, but I haven't had any actual chunks out of tails since the first few days.
Supposedly sibling females are more likely to be fine together if they have been raised together. If you wanted to take the time to get siblings from a breeder, you certainly could to add with your current fish. Personally though, this seems like no guarantee to me and its more fun to mix up the colors
