I believe this is the third or fourth thread you've started all with similar questions. No problem with the questions, we are here to answer them and help you out, but all of this could have been kept to one thread... it's just good forum etiquette!
On to your question. I haven't used plaster of paris before, but due to its thickness and the fact it is water based, I think it would quickly deform a pepakura model, as well as be very, very fragile.
Your questions seem to be relating to the safety of materials quite a bit, so why not try your hand at using eva foam? It's relatively inexpensive and quite easy to work with! There are a ton of threads with builds using this (I have a lot of pics in the build thread in my signature) and a lot of information about where to buy it throughout the forum.
Cheers!