I would think slightly smaller should be fine. The only consideration is if you have chickens with large comb & wattles. I have a rooster who got a bit of frostbite on his comb tips and i've been putting a bowl for him so he doesn't bump the injured bits on the elbow. Otherwise I have no concerns. My Brahma hen sometimes even will eat from the 2-inch (5cm) elbows I put in at first when they were babies. it's a close fit, but she never gets stuck in there.
Probably, but I would think that they might introduce water into the bin, creating a moldy mess.
Yes! I have 2 ducks and 3 chickens penned together. it's GREAT for the ducks. I haven't had any problem with them messing it up, but I put it a bit away from the water so they can't just go right back & forth... Their ducky heads fit in so nicely and no combs & wattles to get bumped like the chickens. My female is a silver appleyard and the male is a mixed-breed slightly smaller (probably part appleyard and maybe campbell or harlequin or something mixed in). Homer & Hesiod both eat from the elbow-feeder bucket just fine.
I don't have anyquail. I can't imagine why it would not work for them, but you might need to alter the depth of the elbow on the inside, depending on how long their necks are. I have 2" and 4" pipes on my feeder because initially the chickens were still little chicks (2 or 3 months old when I first built it). I just added the bigger ones when they got older.
