It's better if the frame has 135mm spacing, but an earlier 130mm steel frame can be modified, if you shout at it loud enough and let it know who's boss (Sheldon Brown gives more details). I shouldn't really advise because I don't know about early 90s Rockhoppers, but I would be surprised if they weren't 135mm.
You just need a spacer if you want to fit a 7-speed cassette to an 8/9-speed hub.