I need help cuz i idk how but i lost my peripheral cable(for fans) and i need a new one. I am wondering...if all 4pin or 6 pin to molex are compatible with all psu . My psu is an old one, the seasonic x750 rm not the rm3
 
^ I think youre misunderstanding.

I assume the op,currently has no molex cables coming,from his,psu , its modular & he doesnt have the molex peripheral cable for it.

If this is the case , & assuming you have spare sata connectors, then sata to molex adapters, while a little untidy , will do the job.

https://www.amazon.com/Manhattan-15-Pin-Female-Splitter-308977/dp/B002LBROAA/ref=mp_s_a_1_8?ie=UTF8&qid=1538898279&sr=8-8&pi=AC_SX236_SY340_FMwebp_QL65&keywords=sata+to+molex&dpPl=1&dpID=41DEHmY3WzL&ref=plSrch