Lots of questions there.....
1) Yes sockeye and Springs sit in different water, but can also sit in the same water too. I saw a few Reds this year on the edge and even inside a backeddy with hundreds of sockeye around also. Reds seem to prefer Depper, faster water while sockeye will come up the edges or slower moving water (seams, backwashes, etc).
That being said I have seen lots of Springs this year coming up within 2 feet of the edge.
2) Springs like deeper holes, i would say 4 feet plus. The like sitting on seam edges and places where currents connect with eachother. They also like fast water too with lots of riffle/top action or will sit in a slower moving ruin as long as there is lots of rapids, etc in a run just above them.
3) Springs like sitting close to the bottom, Most people I find adjust their float till they hit bottom and then move it 1-2 feet up. This will keep your hook close to the bottom and in the strike zone.
Cheers
Dan