I guess to each their own, but I don't like the idea of putting a weaker/smaller fish in with the FH to use as a "punching bag." "o"/
One of my friends would do this for his FH. He would say, "To let the fish Kunbung!"
I prefer to use another FH of the same size divided with a divider to 'excite' my FH's.

That way, the weaker fish will never be injured. I also use mirrors.
If the smaller fish is not meant as food, I feel that letting/watching a weaker fish get beatened up or killed is just too cruel to allow it to suffer in such a way.
Just my 2 cents.