Hi again,
As far as trainers go, I do believe you are fine on rollers. If you're looking for a resistance trainer (ie. not rollers, but a mag or wind trainer) I know minoura (and other large brands) makes a trainer that engages the rim instead of the tire and it can accomodate more tire sizes. I would check to make sure, but I'm almost 100% positive that it can take a 20" wheel. I know one of the trainer companies also make a 20" wheel adapter kit for their line of trainers, but I forget which brand. I am actually anxiously awaiting the possible procurement of a 20" wheel trainer! It was on craigslist in NYC and my brother is supposed to be getting it for me today. I won't get my greedy little hands on it until tommorrow, though.
Juan