I had 4 armatus about 4-5" along with 50 endlies from about 3-6", before I knew it, I lost about 10 endlies (I counted them after seeing an endli tail sticking out of armatus mouth).
The problem is when the endlies swiimming around the mid to upper column, the armatus takes it as food.
Keep the smaller polys out and leave the larger one in there, if they are the same size, you'll be fine. Get the skinnies ones out.
stan