Just to make sure I understand, in the 2nd (and subsequent) households:
-one member of the new couple must be from my new chosen household
**Yes, this is correct
-can there be other members in the new household other than my couple, as long as the couple moves to their own home after getting married?
**Yes, there can be other members in the household as long as the newly married couple are together.
To clarify, the new couple doesn't have to move, as long as they are in a unique house from the previous couples. The only "move" is when you change to a new household and then if the partner has to move in after the wedding.
So, example:
Bill and Mary are your starting couple and get married.
Then you switch to the BFF household. Liberty and Travis get married. They can stay in the house because it is unique.
Then you switch to the Partihaus household. Marcus wants to marry Summer. Summer would have to move in with Marcus for the unique household.
Edit to clarify, the married couple needs to remain in the current house. Single sims from previous households are fair game, but no one should be moving into an empty house.