R5 glowplugs... does it matter which company its from to be a cold plug? OS describes theirs as cold, then Rossi calls theirs very cold... but just in general, would any R5 plug be a cold one?

Every glow plug is different, so yes it does matter which maker you're using. That doesn't mean that a given engine won't run on several different makers' plugs though. It just means that one will work the best.