Doll is in the game, last I checked. One or two of those others might be as well.
As for the P51D (and the B I think) the full 15 slots are full. No more can be added without removing some others. HTC has said if they like a skin enough they'll rotate some out over intervals, but we're talking long long intervals here, and folks don't want to lose a skin they've been using so far.
It's a balancing act.