You could use a 8 wt reel on a 6wt, it just won't be a very balanced outfit. You could also go with reel more suited for a 6 wt and put gel spun backing on it. The down side is that gel spun is it isn't cheap. You could use that reel on an 8 wt but again it really won't be a balanced outfit.
A 6 wt will work well for trout, pinks, beach and smaller river silvers and summer steelies.
A 8 wt will work well for large silvers, chums, and winter steelies.
In reality, two outfits would be the way to go if you've got the budget.
As far as lines go, you may want to consider a multi-tip line. Rio, SA and Airflo all produce these. If your 6 wt is a fast action rod, consider uplining it to a 7 wt line.
Good luck,
Brian