Monos are schooing fish that grow to ~1'. A school of adults would need at least a 300g tank. Unless you plan on upgrading to quite a huge tank, I'd say it's a no-go. Nevermind that a GSP is a voracious fin-nipper & may kill the delicate mono. When your GSP gets up to marine conditions, you can consider a few damselfish or a tomato clownfish.