First of all, your tank sounds absolutely lovely, and I hope you can post some pics of it.
Unfortunately, your tank is stocked to the max, (with the fry it's
way overstocked) and there's no way you can add any more fish, unless you really reduce the number of livebearers you have by quite a few.
All those livebearers put a big strain on the bioload since they are prodigious eaters and poopers. Corys, in particular, are sensitive to pollutants in the water, so keep a close eye on your parameters.
I don't know what to tell you about all the fry (which are probably increasing in number even as I type this) As you see, most petstores have no interest in livebearer fry. I guess the guy who sold all the fish to you won't take any either?
The only thing I can suggest, and you probably won't like: Remove the plants, catch all the fry, remove all the male or female adult platies and mollies, bag them all up and just take them to the petstore you got them from. Hopefully they'll take them back.
Maybe someone else here will have better ideas.
One more thing - never ever take advice or ask suggestions of petstore employees. Mostly they just want your money, and know little to nothing about the fish they sell.