I personally balance it out by not accepting any stray/kittypet in the clan during fall and bare-leaf (cause a clan don't need more mouths at these seasons), and at new and green leaf I just flip a coin. I usually don't allow to add more than one stray/kittypet for season. Plus, if a clan has too much cats, you can take probabilities in your own hands, pick some randomised numbers and kill those yourself in one clean sweep or periodically (explaining this with some disease or rat attack or smth).