Re: Tooth sprocket question...
the smaller the wheel, the quicker the acceleration. you can get away with a smaller gear than the average 26" bike has. in theory, you'll need a smaller sprocket to get up to top speed, compared to a bigger wheel.
like, if a 26" had a 46 tooth, you'd need a 36 to keep up (i'm just guessing, i'm sure there's an equation somewhere that'll tell you actual wheel diameter + sprocket size = speed..)
there's people that claim a smaller bike won't be able to keep up with a larger wheeled bike at top speed, but in the real world, they're wrong.
with a 20", you could run a 30, or even a 28 and still climb hills, as long as your engine is tuned right.
i have a 24" with a 30 tooth, and i've run 30mph+ up 6% grades.