A simple way to drain those molds with small pour holes is to insert a straw and blow in it as you drain the mold. The straw acts as a vent allowing air in to displace the slip.
You can make various sizes of earrings by pouring slip into the various sized keys in your mold.
A gang mold is what the name implies, a gang of molds made into one. Chess sets are usually gang molds.
Core molds are very similar to gang molds except that they have only one pour gate that branches inside the mold
Let air circulate around your plaster mold as it dries. You can put it on top of your kiln or over a radiator to speed drying, but keep elevated so it dries evenly.
One problem with various methods of mold release is they can clog the surface of the plaster, making it less porous, so clay sticks to it more. So mold release should be cleaned off as much as possible. A wet sponge after using your mold is a great way to maintain perfect molds.