Can I assume that the unit that ends up with the single 30GB disc will be stable without it's old partner?

Stable? What, you're worried it's going to go postal or something? I guess I see your point. I know that if I lost my old partner, I'd be unstable, too.

To answer your question: As long as there aren't any duplicate FIDs on the hard disk(s), and as long as you force a database rebuild (from synching), all should be well, the player is designed to auto-detect the number of discs present.