When we have multiple datasource kinda setup, it is recommended to have a master datasource which will have all the ID syncs.
You can select up to 3 cross-device data sources to use with each profile rule. The Authenticated Profile Options are available when you choose Current Authenticated Profile or Last Authenticated Profile.
While creating segment you can pick the master data and while creating PMR as well you can have only master datasource added but during offline dataonboarding you need to mention UserID datasource as the mast datasource and in the TargetDatasource you can provide specific datasources.
I hope this helps, please let me know for any additional question on the given matter.