They're not Service Animals (as covered under ADA), though for the purposes of housing the two classifications are currently nearly indistinguishable.

Just be ready to deal with a more blue collar working class. If

The regulation on employee classification is sweeping across the nation.

If the units are not the primary tenant’s residence, the appropriate use classification is Visitor Accommodations.

As others have said, while the HUD guidelines are typically looked at, it will be difficult for you to establish that you are not discriminating against a protected class (if some of the tenants are children), or otherwise using a pretext to bring the unit back to market rates after eviction. 

I should have stated this earlier but the passive vs active classification I was referring too was at the LLC level where K1s are issued.

As many create a legal entity for the "asset protection feature" and not tax consequences, please comment on How or IF the flow-through/disregarded entity classification impacts asset protection.

Starting in 2018 the new tax cut act removed computers from the listed property classification and can take full advantage of the section 179 bonus deduction EVEN if used less than 50% for the business. 

What is the default tax classification of a single-member LLC?

I would suggest acquiring all of the financials on the portfolio and packaging them into an easy to read list that shows pertinent information (i.e. cap rates, rent rolls, classification breakdown of properties, etc.).