coilovers advice

[NoHachi]

No x kg/mm is x kg/mm, that means that a spring will move 1 mm for each x kg. Spring length is not of influence on spring [u:3abeff90fe]rate[/u:3abeff90fe].

Your feeling comes from the fact that softer springs NEED to be longer because they are softer. If you have a 200kg on a 10kg/mm spring it will move 20mm.
If you put the same weight on a 2kg/mm spring, it will drop 100mm. Its easy to imagine that the 10kg/mm can be shorter, say 60mm, and work OK. But 60mm on the other spring would be way too short, it moves more then that due too the weight, so it will collapse to its block height (all windings stacked).

So spring length is important! Read the link I sent!! Do your own thinking.
In general you want:

That the spring is long enough to absorb all the weight it will see (roughly cornerweight + dynamic load = 2x cornerweight roughly) without coming close to block height.

That the spring is short enough that it moves the spring more then 10% with just the weight of the car on it.

If you don't have helpersprings then you also get the extra problem that the spring length needs to be at least equal to the shock stroke.

After you figure this out, you buy the spring and set rideheight with the collar.