Resin will work great in a silicon mold. People use plaster because it is cheaper. If the molds are complicated and have deep under-cuts you should use something for a mold release. There are a few products out there, but mineral oil, vaseline, or talcum powder is fine . You can cut mineral oil with alcohol and run it through your airbrush if you are doing a lot. Also you can use the AB to poof the excess talcum.
You should probably mix up a small batch of your resin and do a test run or three. It takes some time but can't hurt. I love doing molds but sometimes they don't come off and if you do a big batch that turns into plastic goo or abstract honeycomb sculpture it sucks.
Also keep that little brick mold on hand. I always have too much resin and it's nice to pour off the excess into something that will be useful later on.
I do hear that the Chinese PU Resin can be pretty smelly so keep a mask handy...
Seriously, don't breathe the fumes.... PU resin is the worst...
