with that many marines you could build 1000 pts of each (ba, sw, BT/DA)
and have some vanilla mentors so the mentors could proxy but itd still be clear on what army your playing
so 1500 pts
1000 pts of chosen army (SW) (= grey hunters/ bjorn/ fancy puppy stuff)
500pts of mentors (tacs, devs, stuff thats in all SM lists )