Squeak,

The PSRs will absolutely sync with a drum machine, and most of the latest models (including the 550?) will also trigger a drum machine if the ACMP OUT template is selected in the MIDI utility. Set your drum machine to Ext sync, connect MIDI out of the PSR to MIDI in of the MRT-3. When you press START on the PSR the drum machine should start, when you change tempo, the drum machine should too.

Hope that helps,

Midnite