[CentOS] directory permissions

Hi,

I have changed directory ownership permissions recursively such that
it is owned by username:groupname , where groupname is not the default
group, i.e., username. However, when a user creates a new file the
default permissions are again username:username.

How can I give  ownership permissions on a particular directory so
that any files created in that directory will always have specifc
username:groupname permissions?
Also is there any option that would allow only owner to delete files,
even though group has rwx permissions?

Thanks,
CS.