Dumb Question about cables

[bigb]

I'm replacing all my cables and housings and a new Shimano 3 speed shifter. I noticed the shifter cable is slightly smaller than the brake cables, does it take a different housing or can I use brake cable housing?

edit: OK so I Googled it and it looks like it's a bad idea, but then I went back out to the garage and looked in the box that the new shifter assy came in and lo and behold there is a housing tucked away in the box.

 

[Wildcat]

You found the right part and it works? Make a note of that, and celebrate. That is a rarity. Take a pic and tack it up on your workbench for posterity.
99% of the time, the part is missing or can't be found. What you do find won't work, but looks like it will. You will spend hours trying to get it to work, finally giving up, with cuts and bruises and a warning to "Stop all that cussing!". The only recourse is to buy it new, only available at the bike shop where it will cost twice as much as you have.