Sounds awesome! Can't wait to see it when you're finished!
It sounds like you're working harder than you really need to. I would keep the girls separated, yes, but don't worry about making them see each other. Just make sure they're healthy.
Honestly, I float my girls in zip-loc baggies. This is just cause it's easier to let them float for an hour or two. I do this to see who is flaring the most, and it gives me a chance to see which girls I can put next to each other. This is important, since you have to release them all at about the same time. I would suggest floating fat mary with the other girls, since releasing them all at once is SUPER important. It helps spread out the aggression by easing territory issues.
Flaring is totally normal. In fact, one of my girls flared like mad when she was floating, but when I released her she sort of went into shock since she didn't know what to do once the other girls could get it (I laughed...I'm a terrible person).
Also, make sure to note their personalities when separate as well. The extra feisty girl(s) will need to be released last. You can release all at once, or two at a time.
I'm not sure if an open area will be a problem, but I know that my smaller girls like squeezing into my floaters when they're being chased (even if it's by another smaller girl). They also enjoy sleeping there (I guess it's since the bigger girls can't get to them). I think that a betta hidy-hole would be fine, but if I were you, I would look into purchasing plants online. (:
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