A little over stocking is fine; but that is more than just a little overstocked given water changes are only small and done once every week or two.
I'd be doing at least one 50% change a week without fail if not more than that.
You'd almost certainly stunt discus that are that small with your current WC regime, so first course of action will be to increase the frequency of WCs.
That will most likely solve the issue with cardinals popping off as well, as I believe it has more to do with water quality than any thing else.