I'm a novice mechanic (been doing it for about a year now at a privately owned shop in Seal Beach, CA) and have some questions about derailleur geometry. Tried doing most of the research myself before posting here but still have a couple questions.

We received a Trek Multitrack 750 in for a tuneup. Looks like it's from the late 80's or early 90's, with SunTour components on it. The cable for the rear derailleur was rusted and corroded which made the shifting do funky things. Without thinking twice about it, I snipped the cable at the derailleur and removed it. Only to figure out that it's a cable that is no longer made. I replaced the X-Press SunTour shifter with a 7 speed Shimano trigger shifter/brake lever combo because I felt that was my only option. After everything was installed, the cable tension was super funky when trying to adjust the shifting. Another mechanic suggested that I replace the derailleur and see what happens. Sure enough, I replaced it with a Shimano Altus derailleur and it worked (shifted correctly).

To my dismay I realized I had done this once before. The SunTour derailleur didn't work with a Shimano shifter. After I screwed up this 2nd time, I did some research on rear derailleur geometry and actuation ratios. What I concluded is that Shimano derailleurs move 2mm for every 1mm of cable movement and SRAM derailleurs move 1mm for every 1mm of cable pull. So my question is, are SunTour derailleurs supposed to work with new Shimano shifters? If not, then what actuation ratio do the old SunTour components have? If they are supposed to work, and I'm just a horrible mechanic, what am I doing wrong? I saw on Sheldon Browns website that there are different ways to route the cable underneath the bolt on the rear derailleur. Could I have been doing that wrong? I thought SunTour/Shimano had some sort of compatibility? But maybe that's with the freewheels?

Anyway, any information on derailleur geometry, actuation ratios, and SunTour/Shimano compatability would be GREAT. Thanks!