I don't understand the question? Usually when you are buying shock absorbers or springs, you buy it for the exact car. The length, stiffness, etc, is different. I doubt Golf's shocks would fit the i30.
Unless you are talking about the sport/"aftermarket" ones? In that case, there is a possibility you could use the same ones on more cars, but again.. You usually have to say which car you have, which engine (diesel is heavier so the shocks are not the same), etc.. Once I bought the sport shocks for my previous car - Accent, and apparently I got the front pair for the petrol engine version (which is lighter then diesel) and the car went too low (more then -5 cm in stead of going -3 cm). There are also few other factors, like..the shock absorbers not being mounted together with the springs ->
seperated or
togetherI probably misunderstood your question and you probably know all those things, so don't get my answer wrong.. I just don't know exactly what you want.