Good idea or bad idea? Limiting Tespia to have it?

All you need to do is have it a little closed off and have 3 month seasons or something like that, at the end of each season only extend invitations to the next to people who actually helped and then take on a few new applicants. Eventually you'll end up with a size-able portion of users who actually test things, and will be able to test all the new stuff with everything else.

Also given that you need to test how new content fits in with the old, I don't think it'd be worth the effort having that kind of limited area tespia when you're missing out on a huge number of potential glitches and exploits.