1) It probably depends on how deep the shade is. Really all you can do is try it out. If the tree is using water and growing, you are ok. If it is not using water very rapidly and is not growing, there are problems. I would expect it to need water within three days of being watered or sooner, but you want to check it every day. If it goes a week between waterings, it may not be receiving enough light.
2) This is really just a rooted cutting, so the world is your oyster. You can begin by thinning it out a little so that you can see the existing branch structure. Junipers can be styled succesfully in most any of the classical forms (except perhaps the broom style).
That's done through thinning, pinching and wiring.
You'd want to clean out the interior so that there are a maximum of 2 branches coming out of any one point. Some of the
Junipers in the Gallery might give you some ideas for where to go with it. Don't cut directly through any green branches or they are likely to
turn brown and die back.
There is a little article over on the
TreeBay Bonsai Tools & Supplies
site called Beware the Mallsai. I think it is linked at the very bottom of the What Is Bonsai page, which may be useful reading for you.
Regards,
Matt