Question 1 Sort of depends on if you're getting ducklings or adult ducks where you will know the gender.
With adults I would recommend a minimum of three... And if you plan on having a drake only one being a drake. Folks will recommend having way more females to male ratios.. In my experience with my calls there is no magic ratio to guarantee there will not be overbreeding or fighting. It all just kind of depends on your set up and the temperament of the birds you end up with. In my opinion three is a good place to start. Even if you are going for three of the same gender.. If something happens to one of them at least you still have two.
Being flock animals they do better with more than one.
Being loose in your garden will just depend on how much you are willing to sacrifice your garden.
You would be surprised how quickly those tiny little feat can stomp plants into the mud. And they seem to like to rip leaves off of things just for the fun of it , whether they are eating them or not.
I do not know much about the watering question.. I want to say it would be fine The one point I would bring up is to be careful about splashing poopy duck water on produce You were going to eat.
As far as caring for calls vs. Standard size ducks there isn't a whole lot of difference other than calls can fly, so yeah you have to decide if you want to wing clip or not. And they are small and can wedge themselves under and between things which can pose a problem. And of course they are a bit more at risk from flying predators like hawks eagles and owls. It's better to have a covered run for times and you can't be there to supervise them.
Tiny dux are addictive so there's that too.
You'll start with 2 and quickly wind up with 20. So watch out for dux math.
