Also, I would assume that, in a four power tube amp, only pairs of tubes needs to be matched.I agree with this. I was using KT77’s when one of them red plated. I bought a matched pair as a replacement for the right channel, biased them and everything sounded fine.
With regard to small signal tubes, I always use matched pairs. You can use a pair of unmatched tubes as long as they are from the same manufacturer, same factory, and same generation of that tube type; ie, a NOS RCA 6SN7GTB with a used RCA of the same type which has comparable measurements. Most likely there won't be any difference in sonics with the amp.
Be advised that the used tube will probably start to die-off before the NOS.