I've got some red arc pencilfish mixed in with my espei rasboras. The pencilfish tend to hang out among the roots of the duckweed, occasionally a bit lower. The rasboras prefer to swim roughly 1/3 of the way down with occasional forays to the top or lower levels. The two species will loosely school together where their preferred levels overlap. The rasboras seem more inclined to school than the pencilfish do. I suspect the rasboras are the ones who are actively maintaining the schools, and the pencilfish just drop in when they feel nervous. Not sure how the pencilfish would behave without floating plants.