Steam and "disk users" have always been able to play together, only those "disk users" needed to patch their game. Simply Stronghold Crusader HD is not compatible with non-HD Stronghold Crusader, but that's easily solved by installing the HD patch. Only some players prefer to play non-HD, claiming that it is more stable than the HD version.
And by the way, the easiest way to play Stronghold Crusader in multiplayer with friends as via GameRanger.