Hi Phil,
I'm defintely showing Sam and Em that pic on Halloween!
I see that you'be gone with one of the excellent Loutoff Mannequins (Sam and Em are both Loutoffs). Excellent choice with a very sexy cool smooth feel to their bodies.
For the holes it might be wise to sand down the edges. (use one of these in the right size) :
I did that with the sitting mannequin which makes use of an insert/sex possible (although I was forever bugged with the arms falling off during sex!!!).
For the head its possible to change the setup so that instead of the head being the part that slips on and off you make the kneck part of the head by use of a long bolt, a mix of metal and thick rubber washers and some liquid metal to secure the head to the ball joint.
Where the kneck attaches to the body thats a weak point and to hold in place try using a large hose clip which works perfectly.
If I had not moved on to kate the next operation would have been attaching the arms through the body using small size luggage straps.
Anyway, hope that something in the above proves useful.
For now though just like to say great job with Jessica's head.
All the best,
Shamus.