Artists from other accounts: You can find them in the selection dialog by searching with at least two letters. Contract artists from other customers can be selected if their contract is active and their profile is public. Credit artists from other customers don't appear in the list. You can only find them if you type in their artist name completely and exactly, and only the artist name will be displayed.
Turning a Credit artist into a Plan artist: You can do this at any time, as long as a Plan slot is free in your package. Open the artist in the dashboard and click "Upgrade to Plan". The artist will then get a public profile.
Downgrading: A Plan artist can't become a Credit artist again. So think carefully before upgrading.