Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Platies are social fish, so more would be a good idea. You could keep about 3 in a 10 gallon tank, even with 2 frogs in there already. Get male Platies only. They will quickly over populate the tank, otherwise.
Euthanasia is simple and humane. Clove oil is best. In low doses it is used for surgery on fish, in higher doses it kills them.
If there is still the option of returning the guppy to the store, even if they will only give you an exchange, do that.
I usually quarantine fish for 1 month. I would set up a separate tank, since you already have frogs. If the new fish need to be medicated you do not want to be treating the frogs.