Please help me with spring suff

I have this set of Tein Flex-Z coils with 20k h150 (orange) and 18k h180(blue) springs, but they are too short to have any kind of preload on them. I'm still a little confused about if I should have preload on them or not, and if I should get some helper springs or just longer springs or just run them as is. Any advice would be appriciated. They are for an is200 if that matters. Thanks for all the constructive answers:)