I'd add the sponges, or a sponges prefilter if you are worried about it. I had Emperor 400 filter on my tank and she showed the babies how to swim around the intake areas, I was surpised and not one was sucked up. It's going to take them awhile to get to a half inch size which is when I would consider separating them, and you may need to cull some off and some will probably pass too, I wouldn't worry about a 40 being too small.