I notice that nobody answered the question on the height of the Sofrider's top tube.
Based on manstir99's side view of the Sofrider V2, I notice that the red line is tangential to the spoke face of the rear wheel rim. I took that measurement off another bike's 26" wheel and got the result - 24" (610mm). So the Xootr Swift's top tube is about an inch higher (25mm). IMHO that is perfectly acceptable.
I've ordered a pair of 20" x 2.0" (50-406) Big Apples for my Swift. I'll post my impressions of them on here when they arrive (early June, I hope).
I found
a very long thread with lots of useful info about the bike on
Bike Forums. The rear, Big Apple tyre apparently is quite a close fit at the chainstay. I hope that mine's alright.