Haven't these platies been just about to give birth for weeks? I think you need to relax so that they can as well. Add decor to the tank or a bunch of moss / hornwort and stop stressing them by moving them around for the "imminent" birth.
Check out http://www.djramsey.com/. Dave's got great fish and I think he always has dageti available. We had a spawning pair from him awhile back, but lost the male.
They should coexist fine even though they are both better in larger groups of their own. Yoyos can get rowdy which is why they get the semi-aggressive label, but IME it's just hyperactivity and they don't usually mess with other fish.
I agree with Leave them. IF you develop a snail infestation, something is wrong. And by that, I mean you're doing something wrong. Snails aren't a problem without overfeeding.
Good luck with the peppermints. They are hit and miss and even the hits may leave larger aiptasia alone. Make sure that you ordered the correct species. From what I remember, there are an Atlantic and a Pacific species. One readily preys on nems, one never will. I could be way off though, it's...
I agree. It's aiptasia and they tend to spread. Manual removal usually makes the problem worse. They can be killed with hot water, lemon juice, kalk paste or similar commercial product like Aiptasia-X or Joe's Juice. Google "aiptasia removal methods", there are some good articles out there...