you are at your limit, honestly a bit over I think. Platy are very messy fish, little poop factories. Guppies aren't much better either really. I wouldn't do anymore or would expect some water quality issues.
Do not go with the cory, they are shoaling fish and you need at about 5 of them to be happy. 10 gallons just does not give you that option.
1 fish, 2 fish, red fish, blue fish