The thing to know about ERM is that it allows you to select documents to be put on hold (say, a regulatory issue, or lawsuit, or audit) or 'implied hold', which means 'hold everything I load until I say it can be deleted'.
To answer your first question... In the first situation, yes. Documents without 'holds' applied to them will expire as per your standard schedule. Documents *with* holds will be held. In the second situation (implied hold), you will have to remove the implied hold from documents in order to get them expired.
You're going to want to work with a specialist in CMOD on Z, because the implementation details are tricky, and going back to apply ERM on old documents will take some work - either low-level changes to database tables, or export and reload of data.