If OUs with non-Windows assets are submitted for Scheduled Computer Scanning, those non-Windows assets are passed along to the same scanning procedure that's used for IP Address Range Scanning. Non-Windows assets submitted for Scheduled Computer Scanning *will* be scanned regardless of whether or not their IP ranges are visible under IP Address Range Scanning as well.
It's a little counter-intuitive, but you should be able to use IP exclusions, configured under IP Address Range Scanning, to exclude non-Windows assets from Scheduled Computer Scanning, even if the machines' ranges aren't visible under IP Address Range Scanning. Windows computer exclusions won't work, as they only apply to Windows computers.