GG, I can only give my experience. I get 8 sts/inch on size 2 with KP Felici & OnLine Supersocke yarns, both are smallish fingering weight. My next online order will have some size 1 fixed circs in it to see what my gauge is on these yarns, which I love.
I'd recommend that you go with a length that will give you the amount of loop you like.
Liking lots of loop as Lighting does, I use a 40" for one sock and for 2 at a time I'll go with a 47". I don't know about knitters pride interchangeables, but it seems most sock needles are too small for interchangeable joins. <sigh>
I'm learning the 2 at a time cast on now because I'd like to have a pair finished at one time. Who'd think that knitters can be impatient? :lol:
Good luck on your search for needles, and please let us know how you like the 2 at a time technique with sock yarn.