was wondering if it's possible if i can switch one of my ark server to a rust server?

Sorta? Just cancel the server, then create a new server as Rust. The bill will be the same.

but if my bill is not due till 21st then i'll be charged after 8 hours running rust

Nope. Billing doesn't work that way. You're billed for only the length of time you use a server, and you only get a free trial for 8 hours when you first start your account.

If you run an ARK server for 14 days, delete it and add a rust server for 6 days, then delete it and add a 7d2d for 10 days, you're charged for 30 days service for a server.