There used to be - maybe still is - a fanzine "The Blues Brothers" which was a Rangers/Linfield/Chelsea thing, the links between Rangers and Linfield have always been pretty strong but I think, as has been mentioned in the thread, the Chelsea/Rangers thing grew in the NF/BNP days of the 80s.
To be honest, I think most Rangers fans I have seen comment on this link tend to say they only support one team - Rangers and couldn't give an fcuk about anyone else.
I could be wrong but I think Hearts - who most non Hearts or Rangers fans view as a smaller version of Rangers in a quite disdainful way - have a link with Millwall, I think there was a Hearts/Millwall banner at the Scottish Cup Final just past, or maybe it was their Semi Final, not sure.
Living in Scotland, but not Glasgow, when I was growing up - 80s - most non Celtic/Rangers fans tended to have a liking for one or the other depending on their religion, but nowadays it seems that most of them just detest both teams equally, although I often hear people say they hate Celtic but "really hate" Rangers.
I was a Celtic fan until a few years ago, just gradually began to realise that I didn't "get it" any more and my interest just waned. If I'm being honest, it was a lot to do with the whole Celtic/Rangers circus and the vitriol it encouraged people to vent, just a bit too much at times.
Been to plenty games between the two back in the day, but I wouldn't even thank you for a free ticket now.