A service or assistance animal owned by someone with a disability is NOT a pet. Your “Pet Addendum” does not apply.
Does that mean you want to give up all your rules about pets? No! Take the simple measure of changing the word “Pet” to “Animal” throughout your form. Think about what you want to control–the consequences of ill-behaved animals or neglectful owners. This applies to everybody. The only thing to remember is, you can’t make it more expensive for someone with a disability-related animal than for someone without an animal. No pet deposits. No “animal deposits.” No pet/animal fees. No charging a higher rent because of the animal.
On the other hand, you can require all animal owners to be responsible, to be liable for damage, to maintain renters insurance, to keep the animal under control at all times it is outside the dwelling unit, and to provide proof the animal is current on shots and other healthcare. You can have a system of warnings for aggressive animals, and declare a default if a disability-related animal is too aggressive. Don’t be afraid to control the behavior of animals and their owners, whether the owner has a disability or not.