I have a pair of rear vs commadore ccalpers and a pair of rear s15 calipers is there any reason I can't use the s15 set up with r31 rotors and the vs set up on the front with ae86 rotors? Also what is the smallest size wheels I can get over these?

What struts?

Ae86 or s13 I havent decided what I'm putting in and leaf down back

Should probably decide what struts you'll use before you go any further. With the 86 struts they use a caliper bracket so you could pretty much make any caliper work with a custom bracket. I doubt you'd get anything smaller than a 14" rim over those brakes but most likely 15". If you want fully hectic JDM as ʞ©$ɟ 13" hardpark wheels I'd suggest just using ADM or the better JDM AE86 brakes.

Okay so the vs on the rears will be fine?

Sorry I mean front vs and rear s15 will be fine?

On what diff? Need info to answer questions bro.

Erm its an r31 diff with a stock 3.8? I think but I have a 4.1 out of a pintara I would rather use