I used Simpson Pellet Vent Pro, 4" size, to vent my Keystoker. Really nice stuff, also really expen$ive. But, it's 1" clearance and high grade stainless liner, so it can handle coal or the exhaust from a corn stove. It's also sealed from the factory, so you don't need any silicone.
I just don't know if you have the room for it in your setup. Your 26ga. will be fine, just replace it yearly. I'd be more worried about the clearance to combustibles than anything else. Here's my setup: http://nepacrossroads.com/about2150.html