It sounds like the USB drive doesn't have the drivers to boot the other macs.

The fastest path forward is to simple erase install a basic Snow Leopard and boot the mac into Snow Leopard.

Don't bother with updates unless the installer forces you to get to a higher version than your 10.6 installer level.

Mount the USB and try clicking the package to execute the Lion upgrade program.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.