Seaguar is my tippet of choice on 6 or 7X. It's very supple in that diameter.
Yes, it's expensive, but 6X in Seaguar is way stronger then many other brands. I fish Rio leaders with Seaguar tippet as a rule. Rio IMO is second strongest tippet. Seaguar's worth it cause you can hustle the trout into the net and you don't lose the big one. I've caught several over 20" in 6X on the area rivers. Don't forget to spit on your knots and pull them down slowwly.