Iím trying to replace and old Shimano Alivio rear derailleur with a used SRAM X-7. This is for an old 7 speed mountain bike, but I thought that as long as I continued to use the original 7 speed shifter that the rear derailleur would not matter. However I cannot get the derailleur adjusted properly. If I set the cable on the smallest cog it shifts fine on highest few gears, but does not pull all the way to the lowest, stopping short. If I pull enough cable to get the derailleur to shift into the lowest gear, it will not shift all the way into the highest gears. It seems as if the derailleur does not move far enough. Also the SRAM derailleur is labeled 1:1, but the Alivio has no markings. This is not a stop issue as I can move the derailleur by hand all the way from the smallest to largest gear.
I thought rear derailleurs were pretty much interchangeable as long as the shifter matched the cassette. Am I wrong?