It's probably the stress of the change is causing them to react. I've never done a sorority, but that is my best guess. Just because there wasn't flaring before doesn't mean they wont have to work out the pecking order once they are all together either.
I love that wood, the spindly/branching pieces are so neat looking. I wish I had gotten some like that! I love the underwater tree look, so I would definitely vote yes if you could get some type of moss or similar for the other branches!
Has your tank started to grow in yet? I definitely think you should have more cover in there. There seem to be a lot of bare spot, and not many secure hiding areas which could create trouble.
Your girls are very cute. I hope the intros don't stress you out too much and go smoothly!