I'm not really a duck person, but based on what works for chickens, I would put all the same-age ones together (that's the 3 at 4 weeks old), and have the older one (6 months) live next to them but separated by wire mesh. That way they are all safe, no-one gets too lonely, and they get to know each other so integration will be easier later.
As they grow up, I suspect you'll have trouble with two males and only two females.
If you choose to get rid of the oldest male, you'll be left with a trio that are all the same age, so they should be able to live together with no problems.
If you want to add more females, it might be easier to get more female ducklings very soon, because if you wait longer there will be a bigger age difference between new ones and your current ones.