HI! I like the idea of having the door between the cages, I
hope to build a nice set-up like that one of these days..
anyways on to your questions:
1) is it safe? i am worried about the female putting too much pressure on the babies when going up or down the shelf.
A shelf should be fine, Jeff, the boaphile, keeps his females
in shelf-cages when gravid and they are fine. (plus in the
wild I'm sure they still crawl up and around.. it's more
movement brought on by handeling that can cause damage rather
than the boas moving themselves around) I would place
the shelf in the back, but that is more about your personal preference.
2) whats the best way for opening the cage? sliding glass doors or front pannels that go up or sideways? how thick should the glass be?
I prefer an entire pannel that can go down or to the side,
but either works fine, I have cages with both types of doors.
Not sure about the glass thickness, maybe someone else
can help with that? Probably 1/4-1/2" would be ok? Heavy tho!
Plexi works good too. Make sure there's a lock so the snake
can't push it's way out.
3) what size of hide would an adult female ( 8 feet) need?
a hide is not as important for an adult boa as it is for a
young one. Young boas need to feel secure, but once the
boa is over 6' they are an apex predetor in the jungle and
don't have to worry about too many predetors. Most of mine
don't have hides, rather I place large logs in the cage which
they can squeeze between the log and the cage wall if they
feel the need to be secure, but typically they just coil about
anywhere in the cage. I do not provide a hide for my
gravid girls as I have heard of them being squished sometimes
when giving birth inside the hide.
If you do use a hide, a large rubbermaid turned upside down
with a big enough hole cut in the side would work (even those
jumbo half logs are too small for a large adult boa).
I use empty 12-pack soda cartons for hides once my boas are
over the 4' mark, cheap and easy to dispose and replace.

3b)should i make it a fixed hide or use some sort of container?
I prefer containers just because they are easier to remove
and clean as well as it makes it easier to get the snake out
if it's in the hide... just lift off the hidebox instead
of having to try to wrangle the snake out.
hope some of that helps, good luck and post some pics when
you get it done!
-April