That plexiglass top is super neat! I would say try it with that many holes and if the humidity gets too high or you notice condensation issues (or you get paranoid, haha) you can drill more. Personally - just my opinion - it's always easier to drill too many holes and cover some of them up with cardboard then it is to drill not enough holes and have to add more later.

As for the screen, it's probably fine. Many owners have screen tops. And unless you notice her rubbing or injuring herself on the mesh, I see no reason to change it.