As regards the orchestration, specifically the matter of doubling brass parts with strings, I am not so sure. Wouldn't layering too much together simply lead to a uniform "orchestral" tone color, rather than allowing the different instruments and divisions to stand out in contrast to each other?You'd think so. And this inner conflict is definately something that is found in many experienced composer's works. However, a full orchestral work can have many colors all while still using most if not all of the orchestra. You find this moral dilemma come up often with composer's who are not writing orchestrally. Instead they are writing using music theory, and melodically, usually at the piano. Instead of writing inherently for the orchestra, they find themselves "arranging" their composition for the orchestra. Writing for full orchestra is no small feat. It needs to be worked up to (starting with solo works, duos, then trios, quartets, etc). But once a composer is truly there, than they will find orchestral devices that will actually become the composition. At that point, they have become a pure orchestral composer. Barring that accomplishment, one can only achieve a "psuedo" orchestral composition through the use of orchestration. If this is where one is at, I recommend still using much of the orchestra for their "arranging". it will sound more professional at any one moment, and that will go much farther than giving them more variation but at a lesser quality of sound.
Evan Evans