I've not found the advice on the Purina site or on their products to be that far off from anything I've found at any other reputable source.
On the other hand, the local feed mill is a small operation run by a couple of people who are into horses, as are most of their customer base. They've helpfully offered to sell me whatever they had on hand instead of what I was after, and their knowledge of the products they carry for chickens seems to be limited to knowing which bags have a pic of a chicken on the bag and where they keep it.
Starter, Layer, granual, pellet, grit, oyster ... all the same to them.
There are different formulations for chicks vs. older birds, different formulations for raising meat birds vs. layers. Sometimes it's merely a matter of feed cost/efficiency, sometimes (such as giving calcium-rich layer food to young chicks - don't!) it is a matter of health. Not sure what your original point was, but you can certainly sort out the good information from the bad on the Internet (at least on this subject) with a bit of work and a dose of common sense.
There are many excellent books out there on raising chickens, perhaps you could gently suggest your favorite to the feed mill people to stock, and perhaps they'll take a look inside while they're at it!
I'm considering this with my feedmill people ... one of them keeps telling me how she'd love to get her own chickens ... I might try to get them to split some chick orders with me next spring.
LOL at "what kind of ducks? ... Yellow ones!"