I would ask Yan (@downtube). He designs folding bikes with 20" wheels. I've ridden several different folding bikes, and the ones Yan designs handle quite well (which is something I can't say about most of the others I've ridden).

Also, you could dig through old threads about the venerable Raleigh Twenty. Bicycle theoretician John Forester did some experiments with one, and found the stock fork had too much offset; when he straightened the fork a little, the handling became much more predictable. The threads I'm referring to are ten or more years old, and I don't know how to find them.

Bear in mind that there are two different "twenty inch" wheel sizes, and that a surprising variety of tires are available for the smaller of those sizes. The tolerances, between what geometry works and what doesn't, become pretty small with smaller wheels.