I don't have waterfowl feed in my area, but the local feed store has purina flock raiser. It is 20% protein, and it works just fine for them. I have been giving mine some chopped spinach leaves in a bowl of water, they would rather have that. Many times I go in the room and they are begging with a full bowl of their feed.
Keeping ducklings and goslings together is just fine, it won't hurt them a bit. Just remember, by 2-3 weeks you'll need to cut down the goslings protein level to about 15%-16% to avoid angel-wing, then cut it down again around week 5-6 weeks to 11%-13% protein. This is for the goslings only, so you may want to seperate them at this time. I don't think the 15%-16% feed (basically chick starter) will affect the ducklings, but I wouldn't switch them (ducklings) to the 11%-13% feed at all. 11%-13% feed is basically chick starter mixed with grains of some sort. I believe barley was one recommended grain, can't remember the other one.
As for the greens, basically any dark, leafy veggie is good. You could even pick dandelion greens out of your yard, provided it is chemical free. Just be sure to chop them up kind of small and float the greens in their bath or a bowl of water.