Not sure about the 2 breeds and if that could be what is going on, but I do know that leg weakness can be from not getting proper does of niacin in the water fowl diet. What do you feed your ducks? Most of us have a few choices when we get our ducklings Chick starter grower or All Flock or duck starter. Not many of us can get duck starter which have adequate amounts of niacin since it is formulated for ducks. So we feed chick starter or All flock. Of these 2 all flock is better since it is made for all poultry. Some ducklings and goslings can still be niacin def even if on All flock but not as much as if feeding straight chick starter with out supplementing. Best to supplement with plain niacin 150mgs crushed and added to 1 gal of water or using Nutritional/Brewers yeast and adding 1Tab per cup of feed till ducklings are at least 10 weeks of age. Longer if they are already close to 10 weeks and showing symptoms.