"My hen is making a consistent 'cackling' noise, is she in pain?"
Nope! That sound means that she is excited to lay an egg! Hens are pretty consistent in their vocalizations, and will often mirror one-another.
"How do I add chickens to an existing flock?"
Existing pecking orders can be hard to break. Roosters that did not grow up together will very likely fight, but hens (or lone roosters) can grow to like one-another.
Always be present when introducing a new chicken to break up fights and calm hens. They should see one-another before having physical access, usually through a fence.
"How do I help a chicken that won't eat?"
Not eating is a definite sign of sickness. Most likely some form of crop ailment.
Place food nearest to where the chicken can access it, locking others out of the coop temporarily.
They are not picky, but might prefer soft foods like cut fruit and scrambled eggs.
If the chicken won't eat after a day, administering medicine will be required (per vet instruction). This is an easy, but delicate, process of drip-feeding using a pipette.
It is pressed to the back of the throat to prevent drowning and gently massaged in small doses.
"How will my chickens know when and where to roost?"