Regarding changing the rules and different rules for different apartments within the complex: it depends who owns it. If it's a management company, they're less likely to have different sets of rules for different apartments, and they're less likely to change the rules mid-lease. If it's a private owner, I've seen more than a few that allow this tenant to do thisthisandthis, but not allow other tenants to do the same things. You can't assume all complexes follow the same protocol for rules or have the same terms in their leases.
That said - if the lease which you have signed and are currently under has no mention of a pet policy, whether for or against, I think you would have a tough time arguing it in court, especially since you stated you knew of the rule before getting the pet.