Hi, Hatrick!
I have had Runner ducks for two years and Buffs for two months. I am smitten!
They are a bit of work, but I find that when I set up their environment well, it's less work. And they are fun and full of personality!
Dolly1218 makes some good points.
Based on my limited experience, and what I have read here on the forum, I would say think about the recommendation that one drake needs to have at least three ducks, sometimes six. Now as a counter point to that, there are people who have pairs. Perhaps someone who has pairs can tell us how they get it to work, because there are a number of Duck Forum denizens whose ducks have had terrible injuries from drakes mating with them too much. For myself, I'd have more than one girl per boy.
The aquarium may work out, at least for the first two weeks. What I found with the four foot long Rubbermaid tub I used for our runners, is that it got pretty humid in there pretty quickly. Ducklings cannot neatly sip their water. It dribbles, and they can't just drink it, they have to play with it. Figure out now, before you have the ducklings, how you are going to keep water and food before them 24/7 for at least eight weeks without ending up with a smelly, unhealthy swamp for a brooder.
My solution was frequent (sometimes five times a day) changes of bedding, a splash catcher to go under the water and catch a large portion of the water, and probably a few other things I cannot remember now after two years. Please don't let yourself get frustrated. Read up on the forum about how different people manage. But with an aquarium, humidity will potentially be a serious health issue. They need to be kept away from drafts, yes, but they splash so much I would be concerned about there not being enough evaporation.
Oh, speaking of evaporation! I test drove the brooder before their arrival, and easily kept the temperature at 90F, the temperature to keep ducklings their first week. The morning after they arrived, I checked and the temperature was between 75 and 80F. Yikes! The evaporation from all the water they splashed pulled the brooder temperature down about ten degrees. I fixed that problem by adding another heat lamp. I used ceramic lamps left from a pet snake that had passed away. Those are less likely to shatter when they get splashed.
In addition to water for drinking, they will need to be able to wash their little heads frequently, to avoid serious sinus and eye infections.
When I read "free range" I think of unsupervised outdoor time. Not at two weeks, I wouldn't. If you mean closely supervised outdoor time, if the weather is cooperating (the temperature is above 75F, it's sunny), and you are right there to make sure they're not getting chilled or harassed by other animals or getting stuck between two twigs or trying to swallow a roofing nail the carpenters left in the lawn, then sure.
Many people let their ducks and chickens run together. Some number of them work out fine, but some have reported drakes hopping on and injuring chickens. That's difficult to predict. It can be done, though. But watch carefully if you try it. Also, the ducks' water habits often annoy chickens.
How many depends on how much space you have for them. Storey's Guide to Raising Ducks has some suggested minimum areas, and I think 25 square feet per duck if their forage area is limited. I have 160 sq. ft. of day pen plus 64 sf of house and porch for the ducks, that is where they are when I cannot watch over them. But we have an additional over 2000 square feet of garden area they can roam in, with supervision. It's all fenced. We have a long list of predators nearby.
The tub sounds like a fabulous idea! and Dolly1218 rightly pointed out that you must make sure it is very easy for the ducks to get out of the tub. They'll need steps or a ramp. Slick straight sides can be impossible to get out of, and ducks can drown by becoming exhausted trying unsuccessfully to get out, then collapsing in the water.
The Rouens sound wonderful! I like my smaller breeds, but Rouens get some good press around here.
Aside from the water management, remember ducklings need about three times the niacin chicks do. They are sensitive to too much humidity, fumes (like cleaning fluid), moldy bedding and feed, and like chickens can get bumblefoot.