I'm new to seramas, so I can't answer most of your ?s, but I know that they can do well in a mixed flock
This is my little serama "Noodle" with a lav orp behind her
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During the summer, my DD kept the bantam chicks in a tractor during the day (for hawk protection) & they slept in the coop with the rest of the flock at night. Other than having multiple feeders/waters hanging at different heights, it's no problem keeping them together. When chickens are hatched & brooded together, they tend to stick together.
Below is a pic of Noodle & her "sister" English Orpington. They looked like twins as chicks and remain best friends. (The orp hatched late & seemed a little weaker, so I put her in with the bantam hatch. Although her biological orp siblings did not pick on her, this big orp kept hopping in with the bantams..... or worse, sitting just outside the chicken tractor in the middle of the yard.... trying to stay close to her little buddies.
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Because we have extremely cold winters, I set up a separate little coop on our porch for the seramas & other bantams. I've heard they can handle some cold, so the wall panel heater goes on at 35'F to keep things a little above freezing. This is my 1st winter with them.