From a GEDCOM persective the RIN is an actual tag.
Defined as: AUTOMATED_RECORD_ID A unique record identification number assigned to the record by the source system. This number is intended to serve as a more sure means of identification of a record for reconciling differences in data between two interfacing systems.
To transfer via GEDCOM a true RIN, 1) the source program in this case RootsMagic must produce and tag RIN for each of the records that it exports. 2) the target program (mac) must be able to read the RIN tag and place it in its database.
Some programs consider the RIN to be equal to the XREF_ID for the record and do not generate a RIN tag, whle not really correct this can be OK. Some programs that import a GEDCOM keep/maintain the XREF_ID as provided in the GEDCOM other do not, and generate their own XREF_ID.
An XREF_ID is NOT an RIN, GEDCOM defines the XREF_ID as: Cross Reference ID or Pointer that must be unque within the transmittion. A pointer stands in the place of the record or context identified by the matching xref_ID. Theoretically, a receiving system should be prepared to follow a pointer to find any needed value in a manner that is transparent to the logic of the subsystem that is looking for specific tags.
Personally I think that the XREF_ID has no value to the receiving program other than for locating (following) additional information for example the children in a FAMily. So once imported the XREF_ID can/will change and is therefore not a sure way of identifying a person.
THEREFORE: Your conserns are valid. The exporting/source system may not produce a true RIN, the importing/target system may not read/understand the RIN tag. Both systems may depend on the XREF_ID as a simulated RIN when displaying a value on their screens and reports, which may not be honored in the same way by both systems. If you can in your source system I would first export a GEDCOM and see if a RIN tag is produced. A FAMily GEDCOM would look like this:
1 [XREF_ID] FAM
2 RIN [AUTOMATED_RECORD_ID]
An INDIvidual would be the same except the FAM would become an INDI. If this is produced then you are half way home. If the target/import system can read this tag then you are good. If this does not occur then you must experiment to be sure that the XREF_ID is not reassigned when importing. Many systems wrongly use the XREF_ID as the RIN when displaying. This is OK in a closed system but not when you share or transfer data via GEDCOM.