You'll be dealing with reasonable adults, if you screened well.  

The tenants are an older couple with an adult son who is disabled and in a wheelchair. 

There is no inventory around Seattle but I will buy in the South and South East states.3) I agree that you can't change adults for the most part.

So, maybe two adults and one kid, vs two adults and at least two kids.And families with extended family living with them, are unlikely to be looking for a two bedroom, one bath home.So, my vote is to leave it alone.  

I also make the tenant identify who will be occupying the property and the lease states that only the identified folks can stay on the property (all adults get screened, even if they aren't signing the lease).  

You have a contract with two adults.  

My rule on the above is every ADULT in the household must fill out an application.

B: 2 big dogs C: can't seem to grasp the concept of "your application is incomplete"D: four adults (couple, In-Law, and aunt) and 3 kids total.

I told each person that came that all adults that plan on living there needed to submit an online application no later than an hour after the showing, and that if there was already an application on file, that I needed an email confirming that they were still interested after touring it.I then was able to filter through the applications to see who met the minimum screening criteria- 1. did they fill out the application in it's entirety, and 2. do they make the minimum amount of income required.

Interestingly, in our country's housing laws, children under a certain age don't count as people- so if your code department limits an apartment to four people and the expecting mother in a family of four adults has a baby, this family would still be considered four people.