What are the specific restrictions on downgrading an iPod Touch to previous iOS versions?
(I'm not certain of this answer -- it's a somewhat educated hunch, based on using iPods for 10 years and Apple products for 25, and looking at wikipedia.  If anyone knows from actual experience, use that answer instead.)

In general, I would guess that you can downgrade an iPod to the version of the OS that it was released with, but not earlier.  So, for example, an iPod Touch 2g could probably be downgraded to iPhone OS 2.1, but not 1.1 (which wouldn't really matter since there was no App Store in 1.x!).  Each iPod Touch gen-N ships with the N.1 release of iOS, about a month after the corresponding iPhone ships with the N.0 release.

An iPod Touch 2g should run all versions of iOS for the iPod Touch that have an App Store, though it doesn't support all the features of iOS 4.  The hardware differences between iPod models might be a bigger issue for testing, e.g., older iPods don't have a built-in microphone.