I put my short NeoAir the bivy and pushed it down to the footbox. It had the affect of stretching the bivy so taught because of its width that there was no room in the footbox even without a bag.
However, when I scooted the NeoAir up, to where it stopped 17" from the end of the bivy, I found that I had just enough room with my Marmot Sawtooth (~5"-5.5" of loft) The fabric was taught where the NeoAir was by my shins, but it didn't seem to be affecting the bag's ability to loft. Where the NeoAir wasn't, in the footbox, I had plenty of room to move my feet around.
If I scooted the NeoAir farther up, and left 24" of space between the end of the pad and the bottom of the bivy, it was ideal. Plenty of room in the footbox, and no taughtness in the bivy. I was able to pinch 3-4" of extra fabric around where my knees and shins were while inside the bag, allowing me some space to move around (or side-sleep) without compromising the bags loft.
So, as the Ptarmigan is 87" long, and the Reg. NeoAir is 72", you might have some issues.
But I personally would feel comfortable using the 60" NeoAir in the Ptarmigan, as it would leave 27" at the bottom of the bag.
I will note that Greg, who sold me the bivy, commented above that he found it too constrictive with the NeoAir. I spent 5-10 minutes in the bivy experimenting; if I remember correctly, he spent several hours.