keeping ducklings w/ older ducks?

hi again,
in thinking about housing for our ducks, i am wondering whether ducklings once out of the brooder and ready to be outside, can be kept w/ an older flock of ducks? i am trying to come up w/ a design for a pen and this is something i want to consider for when we inevitably get more ducklings one day. or would it be better to have a separate pen and run area for ducklings until they mature and can be kept w/ older flocks? is there a better way to introduce ducklings to older ducks?

thanks!
lisa