I have a 10ft round above ground pool dug into the ground. 1.5 years ago I dumped in over 50 cheap feeder goldfish. Followed by 2 turtles. 0 mosquitos seen, but there were water beetles. The 2.5ft depth allows everyone to dive and kept the turtles and fish unfrozen despite a record cold winter. (The ducks have recently re-discovered the now 5" fish and its an effort to eat them, causing much dropping and stealing and running.)
They were wary of the sharp edge drop-off, so you might use a sloped edge to encourage a specific entry point. They also love standing on top of the fish cave to bathe so I recommend some high spots just a few inches below the water surface. No plant lasts more than 15mins, so cage anything you plant.
I upgraded the pump and filter, but its still not enough and the water is green. Any expansion would include a wetland rock/gravel/perforated pvc system and a feather skimmer. Add a low spot to pool vac/drain sludge. You can't go wrong with over filtering.
Currently 18 ducks, 2 geese. Last year was 12 geese, 8 ducks.