Well, I have PMed Thomas to see if it would be alright for composers to omit, if desirable, percussion and piano, and possibly the saxophones.
But, you should really try to learn to write for piano!
It might be useful, down the proverbial road, like if a famous pianist tries to comission a concerto from you, and you have to say no because you always shyed away from writing for piano!
P.S. You seem to view your compositional problems as things that can be stepped around, or hidden, which is usually not really conducive to bettering your compositional skills. I say if you have trouble writing for a particular instrument, write only for that instrument for a period of time, to widen your knowledge by application. Just a suggestion, of course.