Platy are more group fish than schoolers. They like to be in the group, but sometimes they like their privacy too. They're good active fish too, and it is recommended to have 3+ of them per tank. You have seven, so you're good and could probably get a couple more in that tank so long as you don't get too many other fish. =p
It's not exactly "Nature" taking its course when they're in an aquarium, no matter how big it is. I hate the idea of caged fish being allowed to murder the trapped babies. It happens regardless of precautions taken though, but I'd suggest getting an in-tank breeder net, they're not very expensive and go in the tank, some float in there. Either a net or one of the plastic boxes with a divider. If your fish does ok with being moved into one of the plastic ones with a divider, that'll be ok. But if you do that, don't put her in there more than six hours or so. If she doesn't give birth, it's best to let her do it in the main tank over risking her getting too stressed out, and then catch all the babies you can to put into the box to raise them, it's not hard to do. They can live in there for a couple weeks before they're big enough to go into the adult tank. You can also sell the babies, or just plain give them away to anyone or any petshop willing to take them off your hands. It's a waste of good fish to trap them and allow the parents to kill them(the ones that atleast try to save some are good in my book, even I know you can't save them all, but intentionally pulling the survival garbage is cruelty), and very unfair even if they had a 100 gallon, well-planted tank. In the wild they have miles of water to call home and escape in.
Also, good tank mates for platy...Hm, you want a bigger fish too? x.x Let me think. Platy CAN get nippy, so you probably don't want anything with flowey fins, and nothing that would be aggressive and bite on or try to kill them either, but something that can take care of itself with the little nippers...I never added other fish with my platy aside from Pleco, shrimp and mollies. But my aunt used to keep a Blue Gourami with her mollies, which have similar attitudes to platy. But I am pretty sure she got lucky with that, because some of them can be aggressive. Just like with Betta, it'd kinda be a rare luck of the draw thing to get a normally aggressive/semi-aggressive fish to go well with everyone. Hm, I am not sure if I should say this or not, but Rainbow Fish get about 8 inches long or so and are very peaceful. I am not sure the tank is big enough...I haven't done big fish before myself, so I wouldn't know what they need. I do know Rainbows are schoolers though, but fairly peaceful and pretty fish.
Or you could always go for the ever-placid big fancy goldfish. They like cooler waters though, so you wouldn't want it above 74-76 degrees in there, 74 is probably the minimum Platy should have, 76 being preferable compared. Petshops say 72 for mollies and platy, ignore them and their info. lol
I don't really have any great ideas for you on a bigger "Centerpiece" fish to go with them, but you can look into those and others named. You could always go with something like a male sailfin molly of some sort, they can get about 4-6 inches long and are gorgeous, get along with platy...and I hear they can breed with them, some argue it's impossible, others swear by it. I have a male dalmation lyretail molly who is about four+ inches long, he's pretty much my centerpiece in my tank of mollies and they get bigger than most if not all platy. lol This is what he looks like, if you're interested in how they'd look:
He's the pride of my tank, I think something like that would look good with the colorful platy, and go well with them to boot. They're no more aggressive than platy can get, occasionally bully fish happen for all types but it's not too common. They eat the same things, can be in the same kind of water well, they're hardy, and they all eat the same stuff. They also both like warmer waters, though platy usually like their max about two degrees below what mollies do. lol If you keep your temp at about 77-78 degrees, they should all be perfectly happy.
Other than this, I don't have too much to help you with, sorry. =(
Your tank is probably mostly cycled by now to boot, at first it may have been stress causing your platy to gobble at the air, and water being way off from what they're used to, but they're hardy fish. I don't much like the idea of risking my fish like that, but it does lessen the chances of fish dying when you add to the fully cycled tank(That happened to me with my second tank, I cycled it without fish and made sure all the levels were where they should be, then I added four mollies and three died within two days). A lot of times if you cycle the tank without fish, they can all die when you add them. But I still hope you at least cycled it for a few days/a week before using them to help, they are ahrdy fish though and can take a lot. Extra water purifier helps, but most likely it was an oxygen and stress issue for them at first. You didn't want anyone giving you grief over how you choose to cycle your tank, and I am not trying to. But if you really worried about your fish, you would know putting them in an uncycled tank is both dangerous and stressful to your fish and that caused your issues and the death of your mollies. However, mollies are also very hardy fish, I'd blame the provider, not Walmart. =p Recently most of the stores here stopped selling fish because they found out their providers were intentionally cutting corners and giving them sick or neglected fish to save themselves money and had been for a while before getting busted, I always blamed them before I found that out too. ^_~