Hello Cyara and welcome!
Ummm here is my 2 cents worth...
What would be the best size?.... comfy but snug
As you are looking at dual purpose and somewhat heavy breeds, I would not go less that 4 sq ft per bird. By the way...you will love the Australorps! Of my 31 hens, 8 are BAs. They are great girls.
How high must the roosting pole be off the floor?
You can stagger them like steps but I would start at 18" - 24" above the bedding level.
Can the roof above the roosting pole be low... I heard it stops them crowing because cannot stretch up high so can't crow. Is this true?
No sadly it is not. You can put them under crates and in doggie kennels and they will still crow. I would want a couple of feet between the roost level and the roof...hot air rises, so you want to give room for ventilating without letting them get a draft.
How thick should this roosting pole be?
The best roost is a 2 x 4 with the 4" side up. Gives them chickens plenty of room to settle and sit on their feet. they do not grip poles and such like other birds, they sit flat footed predominately.
Should they be housed individually or will they be happier together in a group? What number is best?
Group them. they are social and flock animals. The number depends on your personal preferences. I have 31 hens and 1 roo in a 12' x 12' coop. They are in the run or tractors during the day and good weather.
Must food be available at night or only water?
It is best to keep both available 24/7. they can eat before you ever get in the morning. I noticed in one of your posts that you did not want to go out at the crack of dawn, so leave it available to them and they can start without you in the mornings.
What is the best way to give water at night?
A poultry waterer would be preferred. Make sure it is large enough for the number of birds you have. be prepared to fill it in the morning and again at night.
Other insight for you...using apple cider vinegar not distilled and not white nor apple cider, helps with digestion and internal parasites to a degree. Mix 1 tsp per gallon of water. I would not use it in the water if you have a galvanized waterer.
With your predators, make sure you have hardwire cloth or chicken wire that come down the sides of the tractors and out away from it so that diggers cannot get inside and larger predators will not be able to flip it over as easily (if your tractors will be small portable type).
Good luck with your future chickens.
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