I've been using Rio's Switch lines and their new Switch Chucker as "do it all" lines on both Switch and single hand rods for a while now and I like them quite a bit. I use the MOW tip system for changing between full floating and various sinking tips. This is as close to a one line system as I've found for quick tactics changes with minimal fuckery.
I usually overline my rod by at least one weight for a little better Spey casting performance. The chucker casts pretty true to weight though.
Some of the confusion that can arise with regard to fly lines is a result of mixing and matching terms.
Starting at the fly working to the other end of the line, it seems like the common nomenclature for the parts and pieces of any given line system is as follows:
-running or shooting line
Lines like a weight forward floating would have the head and the running line incorporated and may or may not work well with tips depending on the configuration. Many Skagit style lines come with head and running line built together while others connect tips, heads and running lines loop to loop.
As Stonefish mentioned, there are lots of options for running lines out there. Mono or braid are common as the low diameter shoots very well, but I find them hard to hold onto and manage. I like the running lines made by rio, there are two diameters and I find them easier to hold onto.
I am still not a cop. EZ Thread Yarn Balls
"I don't care how you catch them, as long as you treat them well and with respect." Lani Waller in "A Steelheader's Way."