Yes they can be used for nymphing, don't treat with floatant, or partially treat what you to float or just sink from the tippet connection. is far as 6' to 9' just add 3' of 7x tippet. The floatant I like is the stuff that came with my Thibault silk line, but you can use Mucelin (red tin) or fly floatant.
Are you making your own or buying them?
Pete