Six months is probably a bit young to put training pressure on a pup (except for those that are real prodigies), but I don't see anything wrong with taking a pup out on dog broke ducks and seeing what he does as long as he's not rough on the ducks and you do not end up putting too much pressure on him.
When I have a youngster, I will periodically take it to stock just to see where it is mentally. I set things up so that the pup won't get hurt or overfaced and then just see what the pup will do. But at that stage I'm not trying to put any training on it; I'm just seeing what it will show me on stock.
If the sheep vs. ducks comment is asking whether exposing the pup to one type of stock will be detrimental to training if you then train on another type, it shouldn't be. Just keep in mind that some dogs just don't have any interest in working poultry, so what your pup shows you on ducks might not be any predictor of what it might do on sheep or goats (or whatever). The first time I took Birdie to see ducks, she had no clue what to do with them. I took one of my older dogs in the fenced area and started him moving the ducks around and then Birdie got the idea that she could move them around.
I don't know your level of experience, but I think how and when one exposes a pup to livestock really depends on the human side of the equation (that is, an inexperienced person could do more harm than good, so if you are not comfortable starting a youngster, then I'd err on the side of caution when thinking of taking a youngster out to ducks or anything else).
This is just my opinion, of course.