I think you might find with Greys rods (GRX & Greyflex in particular) that you can use the lower weight lines in any rating i.e. floating, intermediate, sinker etc, but the higher end will only cope with a floating line.
So, for your rod, you'll be able to use #5 lines in all ratings, but I'd stick with a #6 floater as the max for the heavier rating.
Saying that, it sounds like you've got a nice river rod there anyway so you'll not really be wanting to fish anymore than a slow intermediate on a river anyway.
Let us know how you get on.