Rifter,
DMT makes nice tools and you definitely can't go wrong with them.
Since you already have Sharpmaker you have pretty limited use for fine (red) stone and in fact no use for extra-fine (green). Next, extra-coarse (black) stone is basically not intended for knife sharpening, rather for shovels, lawnmowers, at least axes.
So first of all get coarse (blue) stone, if you have some bucks more to spend - get additionally fine (red) one. 6" stones are nice for up to 4" long blades, with longer blades your work might become somewhat uncomfortable.
So I could advise you to recalculate: wouldn't one 8" Duo-Sharp cost approximately the same price as two 6" single-sided whetstones?
Duo-Sharp is double-sided sharpening surface, just choose coarse/fine one...