The material is .080" styrene. The parts will come untrimmed, but the plastic is easy to work with using a exacto and/or dremel.
ABS is a big pain to use, especially here in Florida where humidity can affect the plastic. There is a similar plastic that is more reliable called Kydex. Both ABS and Kydex are a bit more expensive though and I've found most people just don't want to spend the money.
Fiberglass is a whole other thing- I usually sub-out the work when I have to do fiberglass.
Is the Rebublic Commando armor similar to the regular Clone armor? I did Clonetroopers for Disney a few years ago, so I have vacform molds for all that.
Mark