Can I use a Mercury MR52 Reader Interface board from another provider?

Yes. The Mercury MR52 Reader Interface board is the same as the ADC-ACX2/ADC-AC-MR52 Two Reader Expansion Module and is compatible with the Door Controller (ADC-ACC1-17), LP1501 PoE Door Controller (ADC-AC-LP1501), and LP1502 Door Controller (ADC-AC-LP1502).