I agree, it's risky... but not all mbu's have the same personalities. I kept a mbu with 3-4 endlicheri before. If you have multi # polys/fish in the tank, this spreads the aggression level, if any in the tank.
How about an African arowana? or Hepsetus odoe (African Pike)... they are fairly mild mannered fish. I had one with bunch of polys before in the past, they did well together.
