I spent 10 free minutes yesterday routing the cables for the gears to the mechs. As I did this I got to wonder why there are two different types of outer, or to be more accurate. "Can I use the cool groovy blue brake cable outer for my gears?"
I have long understood that the outers are made slightly differently, but have never really bothered to find out why. I went to my bibles of all things bicycle "Zinn" and "Sheldon" . To find that the construction of the gear cables is designed to prevent compression. Yep I'd worked that out by looking at how the sheath was designed. But the compression is to help with indexed gears work properly. Ah might explain why I don't remember having special cables outers for bikes when I was younger and you fiddled with the lever until the derailleur clicked in your chosen gear, (hopefully).
So to answer my original thought yes I could use the funky blue outer, but then the expensive (but a lot cheaper second hand) rapid fire shifters would not work. Needless to say I have used the SIS Shimano outers that I had already.