I've been downloading numerous mods from this site, but I keep finding a bunch that won't open, says that 7zip cannot open the file .rar as archive. Any help?
This seems unlikely. All files pass a virus scan, prior to getting Approved. So, a corrupted RAR file would simply never make it to public view.
More likely, is that either your 7zip is outdated, or your downloads get terminated prematurely.