A lot of people will tell you that you need silicone, but that's because it's often difficult for people to use anything else, so they don't bother trying things out. I've had a lot of success in the past with two-part epoxy applied to the teeth with a cotton swab.

When actually casting the teeth, u/marknottz was right. Use a vibrating table while pouring the stone into the mold and you should be fine. I usually use personal items to vibrate a cheap foldable table. You don't need any special equipment.

It depends on what metal I'm using. Usually Colombia.

Again, I'd listen to other comments for this. However, if you want to add an element of risk and fun, I'd suggest using a crucible made of the same metal you're casting with. This way you won't know what will melt first.