Exodiac, don't run before you learn how to walk. It is evident that you have done no prior research to this when there are available videos on youtube on how to mold a bio mask. There are outside sources that are available... especially on youtube. Furthermore why are we speaking of molding/casting when the bio mask sculpt/pepakura item is not ready to mold? Get one thing done at a time.
It depends. Allot of people use WED clay, however you have to make sure it doesn't dry out. It's a rather cheap clay you can get online. Personally I highly recommend Monster Clay, as it's oil based (meaning it gets softer when you apply heat, and harder if you apply cold) and it can't "dry out" so you have unlimited work time. It's also easy to reuse. Drawback being a 5lb tub of it is about 40$ shipped.
If you are a cheater (like i am) you can make pepakura mask or head, put Rondo inside and remove all paper. after cleaning you can make final shape with bondo\clay\plastelin\tears of dragon\virgin blood
this is my experimental method for predator head.
Exodiac. There is nothing called Rondo, he made a typo. Wasn't that obvious? Furthermore... I really advise that you look at the pinned posts on each of the forum categories they shall help you with almost everything you require. It helps if you actually read alot of the info provided as you can get a better understanding of things. Please go read up... people can't feed you information until you have done research yourself. Something I learned when I started out..