As far as hygiene, your guess is as good as mine.

(Ok, maybe some mild hygiene issues, but my dog doesn't seem to mind!)

How is their personal hygiene?

Dental floss, feminine hygiene products, condoms, diapers, cotton swabs, cigarette butts, coffee grounds, cat litter, paper towels, and other kitchen and bathroom items that can clog and potentially damage septic system components if they become trapped.

I got a call the other morning at 4am from a tenant that was distressed because another tenant had come home drunk at 2am, turned up the stereo and proceeded to pass out in her apt, and none of the tenants could get her to come to the door so they called the police and after an hour of beating on the door they finally woke her up at 4:30.Try a sewer pipe backed up because a tenant flushed feminine products down the toilet and your the one that has to snake the system from in front of the blockage because it's the only way to get to it without draining 200 gallons of sewage on the basement floor, knowing that once you free the blockage you better get the snake out fast and the cap put on the clean out.

I reject applicants all the time for attitude, personal hygiene, stupidity etcWhen you reject you do not give a reason to applicants.

Typically I would coordinate with the existing tenant and have 1 or 2 open houses, however with social distancing and being respectful to hygiene practices that seems impossible. 

Definitely, hygiene and clean apartment is a good habit.

I also mentioned to keep good hygiene, run exhaust fan in bathroom etc.

I always have problems with tenants that flush their feminine items or decide to pour grease down the drain.