Ah, I would NOT recommend making them all in one piece. Just that once they're connected together, it's the same physical thing as if you'd made them all one piece in the first place.
I had all of about three reference pics (standee, pictures on the box of 12" Fett) when I made my armor, and the action figure version looked like it was all one piece. (Side note: I think it would be absolutely cool to see somebody do an "Action Figure Version" Fett. I know it's not anything like the screen used or even pre-pro suits, but I think it's really cool looking).
The whole collar/back piece flexes because it is Sintra, and the hole is just big enough for me to turn my head sideways and slip it through when it's flexed. I wound up popping off all the square-ish bolt things I'd glued on the first time I put it on (due to the flexing). I do have like a size 7 head, so that's a little on the small side anyway. I've remade the collar/back in two pieces, but I can't find the paint that I used years ago and I don't want to have to repaint everything yet. I'll still be using a little bit of velcro, though.
edit: This is the best pic I have of the collar/back armor I'm still using.