singulators are crap for off road use. the spring is not strong enough. as aeroplane suggested, you can zip tie or wire it to the chainstay. i would just use an old derailleur instead. you can run a piece of cable through it... with the stop sitting out of the barrel adjuster and the end clamped like normal... this way you can fine tune your chainline... if your chainline is good, back pedaling or out of the saddle mashing shouldn't cause skipping problems.

on a single speed mountain bike, one cog tooth makes a HUGE difference. i would definitely use the 16. i know folks around here that will run 32:17 or even 32:18! on 26" single speeds, because we have some STEEEEEP ascents here. i run 32:18 on my 29"er, which is just about the same gear inch as 2:1 on a 26".