A 6wt is plenty of rod for Erie steelhead. Tippet size is more important in regard to quickly subduing fish.
Overall weight and balance should be your priority for putting together a heavier tight-line outfit. I don't know what your Redington is like, but I suspect that it may be unpleasant to fish all day as a tight-line rig once equipped with an appropriate reel. Also, due to the inherent stiffness of a heavier line weight rod, it might not perform the way you want unless you are using very heavy flies or weight on your leader. This is particularly true if it was not designed as a euro rod. A big part of the reason that euro rods designated 3wt and lighter are so popular is that they match well with the weight of the flies being used, and the size and depth of the waters typically being fished. Most of the steelhead tribs are small to medium sized streams when compared with typical trout water. Thus the outfits most appropriate for fishing them would be the same as that used for trout if not for the unusually large size of the fish. What you are really shooting for is balancing how well the outfit fishes with how easily it handles a fish once hooked.
As as lines go, I'll just say in regard to "euro lines" your money is better spent on inexpensive floating running line and using that savings toward a better/lighter rod or reel. I've never worried about matching a line weight to a euro rod. I just settle for a diameter of running line that is easy to handle while being small diameter. I think the one I use the most is either .028 or .026".
