Russell's Paradox and Platonic Forms

There's an argument for the existence of Platonic Forms that can be undermined in a similar way that Russell's Paradox undermines the idea that, for every property, there's a set of all things that have that property. The argument goes like this: similarities exist, but Platonic Forms are required to account for their existence, so all similarities imply corresponding Platonic Forms. If this argument were sound, there would be forms that are similar by virtue of being forms that do not instantiate themse