Can I use other molds?
Yes! But your results will vary depending on what the mold is made of, and how well it's made. The finish on the inside of the mold determines the finish on your cast piece.
Silicone molds work well, but come in a variety of finishes - some rough, others seem smooth but will leave your surface looking cloudy/frosted, some are shiny. You'll have to experiment to see what you get. You can make your own molds using silicone putty too, which is lots of fun!
You can use any mold that was made for resin casting, but be sure to follow the manufacturer's suggestions for which mold release chemicals to use. Avoid thin plastic molds (like those made for candy-making), as resin will stick to that type of plastic and you'll have difficulty getting your cured pieces out.