I think Valerian is an unclear example. Because "they" didn't go with it, the
movie was financed and made by the writer/director/producer who was not
a beginning writer. In that case there was not "they"; the script wasn't
submitted and they liked it enough to make it. That was the passion project
of an established writer, director and producer.
I read that, if you're a beginner screenplay, you shouldn't write sci-fi screenplays, because they might be too expensive to make and they will be rejected. What is the truth about this idea? If someone submits a first screenplay and it's very "large"(eg: contains scenes on different planets, many weapons, futuristic buildings, lots of characters), will it be automatically rejected or does it depend on its quality, and if it's good, people will accept it?
In general this is correct. It is rare that a beginning writer has the connections
needed to get an expensive Sci-Fi script into the right hands. Most beginning
writers will write for low budget, independent filmmakers taking time to build
their reputation and their contacts.
That doesn't mean the script will be automatically rejected. It means it's a long
shot. If the screenplay is very good and very marketable and the beginning writer
can get it to the right people it will not be rejected just because the writer is a
beginner.
You've been working on this idea of a big Sci-Fi screenplay for a while, haven't
you? Is it finished?