The mentors are pushing them to design outside of their comfort zone. I don't know why they insist on this because it only produces less than desirable results. Many famous designers have a focused look and attract those who share that vision. You don't see Michael Kors, Channel or Ralph Lauren branching out into Vivian Westwood's territory!
Within their style I think it is entirely possible to sell to all three stores. All of them carry a range of designers who differ greatly.