You won't need anything over 20# but you may as well go with 50# as it's the equivalent of 15# mono. On that 200 size reel I wouldn't over load it with 65 or 80, because then you may run into line capacity issues.
You can tie mono backing to the spool or use electrical tape. I've done both and both work fine. It just prevents slipping. Even on spools advertised as "braid ready" it's a wise idea.
