I've been reading up on mollies and platies because I got one of each in a rescue tank.
Platies and Mollies are prolific breeders. Females are likely to be pregnant when you get them and can have fry for 6 months after just one mating. So even if you want to just get a group of females, you could end up with a lot of fry for a long time. Is that something you want to deal with? Also, if you get males and females (1 male for 2 females) you could be dealing with fry constantly.
I'd return the mollies. Male mollies tend to be aggressive and nippy in a group. Male platies are less likely to fight and might be a better choice.
ETA: I too read that mollies and platies thrive in brackish water. If I were to go buy mollies and/or platies, I'd set up a brackish tank. I'm still debating what to do with the molly and platy I took in. They have been in nasty freshwater since October. Ideally I'd like to set up a brackish tank for them.
D'Argo became part of our family on 4/6/11
His home is a 10 gallon planted tank.
Last edited by luluo; 05-01-2011 at 02:21 AM.