I never did anything special. I have PSP9 installed on 3 different computers using Windows 7 and they all work fine (i even have PSP8 too, but it is pickier in one installation).
As for the gradients, you checked the file location for your X5 to find the gradients from 2018 I assume, but just in case, did you check where the PSP2018 does save those supplies? Maybe they are not saved where you think, and since 2018 can read its own supplies without thinking about it, an accidental change could have happened that would not affect 2018 itself, but only the other versions trying to find them?
Check that and let me know if there is something there.