I think if you want to keep a school of Panda Cories (6x) with your 3x Platies, it should be fine. (this is without the guppies).
If you want to add a couple of Red Cherry Shrimps along with that, make sure there's hiding places for the shrimps. My Pandas don't bother them, and I haven't seen the Platy bother them either.
A Zebra Nerite Snail or Green Horned Nerite (smaller) would be fine as well.
Just so you know... Platys breed like crazy. I started out with 3 late November... a batch of 4 frys survived, then another batch of 6 frys... and I'm seeing another 2 frys right now... so now I have about 15ish Platies of varying stages. Luckily this is in my 36 gallon... and when they start growing out completely, I have a LFS that will take them in if needed.