Is it a good idea to put an old pair of STH12 (non-driver) that was on my last pair of park skis on pow skis? If I change the brakes... for something like 120mm
Yeh, they'll work fine. I assume by your comment that your skis are ~120 underfoot, in which case, your best bet would be to get a pair of 115mm brakes and then bend them out, rather than going straight for the 130s.