One has a great credit score, one has a rich father (Doctor) who pays his rent no matter what, and two have zero credit history, and both have erratic or very recent job hires.I've already rejected one of the four due to no credit history and too short of job history, and the fact that the guy doesn't respond when I asked for follow up info. 

For example, a doctor wouldn't refuse to treat a serial killer, so transitively it would be bad business for a landlord to refuse to rent to a serial killer.Do you agree with that statement? 

I have an emotional support animal, and my doctor has written a letter saying why such an animal is beneficial to me. 

This would have to be sent from a company specific email or fax that I can reference online so the tenant isn't doctoring the form.

That is a type of niche you could consider--and grad students in a doctoral program sometimes stay for up to five years or more.

Greg S. has a good point about requiring the animal be certified and that you receive written proof from a third party professional (medical doctor, psychiatrist/psychologist, social worker) if the need for such an animal is not apparent and connection is not obvious to someone unfamiliar with the applicant and the animal.

That is a whole other issue which as a landlord you can require all the above for pets but also request they show certification from a professional third party(doctor, psychologist, social worker) that the animal is a service animal and the clarifies or describes how it provides this service to the owner.

He has since added doctors / attorneys / etc as clients to manage their properties.

During the summer months if they don't have class, they can actually work full time.J-1 students (exchange students) may not work unless that work is an integral part of the studies, such as an exchange doctor may practice medicine if that's part of the exchange program.Spouses and children of students, those holding M2, J2, F2 cannot work legally, period.But most work illegally if they really do need money, such as working on weekends in a Chinese restaurant getting paid cash under the table.

In the form, I'd put in the definition of "disabled" for an emotional support or service dog (whichever he's trying to say it is), and have the doctor agree that he meets the definition.