Duplex have different zoning that may permit additional units.

Whether the seller was open to a buyer commission or not, I wouldn't put an additional cost into any personal purchase.  

If you own a 2-flat with an additional non-conforming garden unit (basement) being rented, and you're considering a cash-out refinance, how will the lender evaluate the rental income?

In southern CA, even garage conversion ADU additions are typically costing more than the value added.  

We're based in Tampa and, in addition to personally investing, I own a mortgage company that provides financing for tons of investors.

Just one additional option.  

You should always require them to add you to their policy as "additional insured" or "additional interest" and you will be notified if they cancel or change their policy.

Many lenders also impose additional requirements such as owner-occupancy, and/or that one lender make all of the institutional loans in the building.- Valuation: The variety of financing products available as individual tenant in common loans remains limited, and the terms are generally less favorable than either apartment building loans or condominium loans.

With substantial equity in your Fort Worth rental, a cash-out refinance could free up funds to purchase additional properties.

Duplex have different zoning that may permit additional units.