Check the Windows event log on the Windows 7 host. Look for event 1111, which should tell you the name of the driver that the Win XP machines are presenting to the host when they connect. When Windows can't match the driver name on a rdp connection it will generate this event.

I have had luck using the "lowest common denominator" with HP all-in-one printers. I install a deskjet 520C or corporate basic print driver package as a 2nd printer on the client. The Server 2008 machine then can recognize the driver and print remotely.

In your case I think you are on the right track adding the universal print driver. You do not have to erase the existing printers. Just add a new printer to both the XP and 7 machines using the newest Savin driver.