Downgrade to FO tank

Hi,

I'm in the process of downgrading to a fish only tank from a reef tank with
live rock for various reasons. My questions:

1 - Can I keep a cleaner shrimp in a FO tank given that I won't be using RO
water anymore and my water quality may not be quite as high as it was in my
reef tank?

2 - What are suitable tankmates for my 3 inch percula clown in my 30-gallon?

3 - How many fish can I keep in a 30-gallon? I seem to remember a certain #
of inches of fish/gallon.

Thanks.

Maddie