If I remember correctly, SSI allows 2 of the specialties to be from a different agency whereas PADI requires all 5 to be from their own. What about the Rescue course, can that one be used across agencies for the Master rating?
Yes. rescue cert from another agency is acceptable.
From the PADI Instructor Manual:
You may accept divers with certifcations through other training organizations into a PADI course using this general guideline, and after conducting a knowledge and skill preassessment.
Qualifying Certifcation Description:
1. PADI Scuba Diver — proof of an entry-level scuba certifcation with a minimum of two open water training dives.
2. PADI Open Water Diver — proof of an entry-level scuba certifcation with a minimum of four open water training dives.
3. PADI Adventure Diver — proof of certifcation beyond entry-level (at least two certifcations total).
4. PADI Advanced Open Water Diver — proof of certifcation beyond entry-level (at least two certifcations total); proof of 20 or more logged dives; documented experience in deep diving and underwater navigation.
5. PADI Rescue Diver — proof of certifcation in diving rescue.
6. EFR Primary Care — proof of CPR training within the last 24 months.
7. EFR Secondary Care — proof of frst aid training within the last 24 months.
-Z