Yes, but only when the score calls for it. The composer of the piece determines which instruments (and how many of each) are required for the playing of his/her piece.
Not at all. Classical orchestral music is not usually scored for piano, except in the case of piano concerti and other music written specifically for the piano. Many symphony orchestras do not...
No, a piano is an instrument. An orchestra is an ensemble of different kind of instruments. The piano does not have a place in the average orchestra. It may however play with an orchestra in a...