I use my 17 mags for the reloads for all my Glocks, regardless of size. I normally just use the 10 rounders in my 26.
I have both 19's and 26's. My 19's pretty much just sit in the safe. The difference between them and my 17's is minimal, and the 17's have more to offer. The difference between them and the 26 is enough that the 26 makes more sense if you want smaller.
I wasnt real sure about the 26 when I first got it, but now, I always seem to have one along. They quickly grow on you.
One thing I've noticed with the 26 is, the mag extensions pinch the hell out of my hands, both while shooting and reloading, but especially reloading. Make sure you have your pinky clear before you slam that mag home. Same goes for the 10 rounders.
I thought the extensions would make it easier to shoot with, but now that I've had them awhile, I find they actually dontt. You learn to curl that pinky under and its not a problem. Also, if youre looking for a smaller gun, why make it bigger with the extensions?
As I said earlier, the 17 mags work well for the reloads, and if you want an extension, there it is. If you dont have a 17, theres your excuse.