Of course I agree smaller flies with smaller weight rods, that is a gimmie, however I do throw #28's with my 6 wt. if that is what is required. I think rod weight is more an issue for the size of fish targeting. I have landed 24" trout on a #32. And yes you can land any size fish on any size rod, but it can also exhaust a fish to death if going too light.
You can also cast large heavy flies with full flex (bamboo and glass) you just need to slow down.
As I mentioned I have the older TL rod in an 8'6" 4 wt tip flex 9.5. The fact that it is a 4 wt. didn't matter, it is a fast action rod. That same snappy flex as the 6 wt 9.5 tip flex.