Hi Shawn,
Heres what my approach would be:
First, you need to get templates, or models, to work with. I suggest acrylic sheet, as this can be easily cut and is generally less expensive to work with. You can try going to a home improvement store to get some acrylic cut, but I doubt they can do it that small. You will probably have more luck searching for a company that can laser cut acrylic for you. They can meet your exact dimensions and should also be able to get the 45-degree edge angle.
Once you have your templates, you can make your mold. Your post says you would like them all on one sheet, but a 10-inch silicone mold is quite large and heavy. Even though it will be more work and more silicone, I would mold each one of them separately.
To make your mold, be sure to put down the smaller flat side. (You’re putting the pyramid on its head.) You want to do this because once you demold the silicone and use it as a mold, you don’t want bubbles to get trapped in resin in the corners.