Can I restrict the rights of a user to download only the Lo-Res version of a file, even though a Hi-Res version also exists?

A user can always download both versions.

The preview (Lo-Res Version of a file) is generated by scoopa automatically and this second version is next to the original Hi-Res file also available for download. A user can always download both files. But if you create "direct-links" you can choose which file should be available for download.