Will Time Machine delete old backups for other computers on the same drive?
Will (Time Machine) delete the oldest backups only from the current machine's
  backups (the retina MBP), or will it delete the PowerBook's backups
  since those are technically the oldest (even though they're from a
  different machine)?


Time Machine only deletes the oldest backup for the current machine. macOS utilizes the File System Event Store to log each UUID during the backup process. Your machine's UUID should match whatever backups have been done previously for that machine, and only that machine's events are handled. All other events are effectively ignored.

You'll need to thin them out by hand using this tmutil delete command:


https://apple.stackexchange.com/a/55645/5472