Will these work for resporonics nuance air pillow?

No. This is a Resmed product only and will only work with the Swift Fx or Nano mask.

Will this work with Resmed P10 pillows?

I think these are are only meant for the Swift FX, which has different attachment for the pillow than the P10. Resmed website could help with a definitive answer.

Is the headgear adjustable or are there different sizes?

It is adjustable, but since it moves during the night, you don't always get a good mask seal. I have very thin hair and I have both straps as tight as they can be and still get mask leaks. It's very frustrating. I switched to the Bella alternative to the mask, and while it works great for mask seal and for not moving around, it does hurt my ears! I jerry rigged a cushion by wrapping a huge bandage around the strap nd now it is wonderful. Of the two, I prefer the Bella. I noticed online that they are sold in one package along with the rest of the kit. I bought the Bella separately and it was about $22 on Amazon. Worth it to me, but try out the orginal headgear. If your hair is thick it might work fine for you. The top is adjustable with pulling the slip cord tighter and the back is adjustable on either or both sides, since it is a Velcro tightening. Good luck!