Mirrored tooth

I figured out why you're having trouble. When you mirror an object in Blender, the program gets confused about which is the inside/outside of the mesh. Because of that when you mirrored the tooth and tried to use the union boolean it was merging the outside of the gums with the inside of the tooth. I couldn't figure out how to fix the mirrored tooth but I have figured out a work-around.

I downloaded a low res mouth model to make it easier to see what's going on.

At this point both teeth are part of the same mesh, so next we need to remove the original tooth.

Holding ctrl while you right click and drag creates a marque selection to select multiple points.

If there are faces left over after you have clicked delete then you will need to select them and delete them. If there are points or edges left over with no faces you can still select them and delete them by clicking "delete" and selecting "edges" or "vertices"

 Hopefully that helps. There's a lot of trial and error with 3D. If anything doesn't work or it's not clear enough just email me back and I'll try to help.

Cool beans.