All the animals listed prefer different veg.
The ducks will not scratch through the dung piles like chickens--they are a stacking function to 1) eat the fly larvae and 2) scatter the dung pile to not kill the grass in that spot and spread the fertilizer.
The goats prefer to browse on leaves and brush, but will eat grass and weeds if they have to. Not the same grass as the sheep or the cattle--so there is a way to stack all three, but the numbers in the mix are really dependent on your specific vegetation.
We did have some free-range rabbits for a while and we seemed to be able to harvest as many as with the tractors with almost no inputs. But you have to be willing and able to harvest them like a wild rabbit (trap or shoot). We had to depricate the population so they didn't strip the bark off the new orchard trees (that probably died in the drought, anyway)

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"Family farms work when the whole family works the farm." -- Adam Klaus