Hi,
If I am getting the right meaning, you want to have some pre-defined permissions when document is created. the problem here is that the permissions are related to documents and not to document types.
first option is to create some parent document where you will set the permissions and allow only this certain document type to be created underneath it - so the child documents will inherit the permissions from parent.
Second option is to use
custom event handler and set the permissions when the document is being created (
tree node handler).
Best regards,
Juraj Ondrus