When you see a PSI number on the side of a bicycle tire, please append on there the words "Maximum Recommended Pressure". You don't have to strictly inflate tires up to that number always and forever. I think that's a big misunderstanding among many bicycle riders. Instead of 100 PSI, try 85 or 90.
Having said that, with a 20" (406) front and 26" rear tire, you have lots of options. I would try lowering the pressure a bit first, but if you want to swap, any BMX street tire with a decimal (20 x 1.75, 2.1, 2.5 etc.) and MTB tire on the rear will work. Of course, some will be faster & more flat resistant (and expensive) than others.
On my RANS Rocket, which is a dual 20" bike, I went from harsh riding but bullet-proof 70 PSI Schwalbe Marathon Plus tires to a Kenda K909 20x1.75 40PSI up front, and a (bit heavy) Kenda Kid Block 20x2.125 50PSI (60PSI max) on the rear. Both with Mr. Tuffy tire liners. I may be a tad slower, but the ride is way more comfortable. IMHO, running a 20" tire at 100PSI is nuts!
Last edited by Recumbomatic; 06-25-13 at 01:53 AM.