Was that a rule that got put into place after or was it already there when you moved in?
I had something similar happen at my building. So many people were bringing in pets (Mostly dogs, pitbulls) that they had to inact a policy where you couldn't have pets anymore.
Only people who are excempt from this rule are the handicapp people like blinds and elderly that depends on them on a daily basis.
There are also some cases where your dog gets grandfathered allowing you to keep him due to the non-existent pet clause.
Anyway, if you brought them cats in with those rules in you either pay up or get evicted. No way around that.