What I have done...not necessarily correct, but it worked for me.
Do I need to put a layer of glass between the layers of coosa/penske (mat, 1708, straight cloth), or just thickened epoxy?
I did put a layer of 1708
After putting down a layer of 1708 (mat side down) do I need to put a layer of mat (epoxy compatible if I can find it) on the cloth side of my 1708 before putting down the next layer of laminate
No(if the next layer is not 1708, but is coosa/penske or outside skin)?
No
Is 1708 sufficient for tie-ing together old skin and existing hull (after a nice 12:1 scarf)?
Yes as long as you stagger the layers
Should I finish with a layer of straight cloth to make fairing a bit simpler? What weight?
I did not...I just sanded the ruff stuff off first, and then started the fairing process.
Some others may have stronger opinions and if they do they will chime in.
Again everyone has there "own" way. This is my way.