Core does not have non-constituent records. Therefore, if a user profile is set up in Core for a spouse, it will come into ConnectRE as a family member. However, when ConnectRE searches for matches, it also searches relationship records, so the user will be able to link the two, and the non-constituent spouse record will be upgraded to a full constituent record.
Note: This is only if the Add each parent as a constituent setting is marked in the ConnectRE settings (recommended). If the school has the Only add one person in the couple as a constituent setting marked, depending on the gender selected, the non-constituent spouse may never enter the plugin.