First, are you going to be “copying” somebody elses work if you cast
these items? It sounds like you would be and at this point I would
tell you to stop unless you can get some sort of signed agreement
with the originator or owner of these designs.
To actually answer your question, you cannot plaster cast a plaster
item so an RTV would be necessary to get a wax.
Again, I REALLY dislike folks “ripping off” other artists work,
so if you do not have permission from the owner of the copyright on
these pieces, DON"T COPY THEM. If you are an artist, make something
that is “your own design” and not a copy of the piece(s) you are
interested in, and use it/them.