The problem with having too much animals alive is that if you have too many other animals won't spawn.
As far as I know for each player there can be 15 animals, so if there are (just an example) 2 players online, the first having a massive farm with over 20 animals, the second having 5 animals then it will be really hard to find naturally spawning animals. Because then there would be only allowed to be 30 animals, 5 animals would wander god knows where and any attempt to find animals would not give any results.
Why I actually started enforcing this is because of the massive chicken farm Samiak_ had, 10 chickens? Ok. 50+ chickens? Not ok.
You're wrong, animals are not affected by the mob cap. Animals will still spawn with normal spawning circumstances (level 9 light + grass), however, it's uncommon.
Why would you start enforcing a rule that you just made up, and has this ever been a problem for us? Of course not.
You're not thinking straight about enforcing these "rules". First the dragon eggs, now this? They are insignificant to the well-being of the server and you're doing more emotional harm than good. The server is fine how it is (for now).