Joined: Aug 2010
Posts: 12,727
Likes: 2,105
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
The hub will spin faster than the designer planned if you use a full size wheel hub on a 20 inch wheel. If you flew down a mountain at high speed there would be a bigger chance of blowing out electronics from overvoltage.
Some hubs are sold in a 3 watt or 2.4 watt version. The smaller wheel would probably work better with the lower capacity hub.
Have you asked on the folding bike forum where most of the bikes are using smaller wheels?