I've had success with 2 short-term rentals, and wanted one more in relative close range to work and life.

I am willing to be dollars to donuts that most of your Section 8 tenants are able-bodied humans intelligent enough to get a job and pay their way through life, but they somehow manage to milk the system.

Google search gives me these super wide ranges and mostly talking about adding a bedroom in a SFH, which I'm not sure if applicable to this situation of "moving/subtracting a bedroom from one unit and add it to another unit in a duplex". 

As mentioned above your new rate should actually be 6-7 range not 8s so shop around.

Investing in multifamily properties in Philadelphia is popular due to the city's steady population growth, job opportunities, relatively affordable property values, and solid rental demandPhiladelphia’s multifamily market offers a range of opportunities, from high-end neighborhoods to up-and-coming areas with great cash flow potential.

The bigger the property, the different the agent on the commercial side.Multifamily in the 10-30 door range 

In my market I think $2,000-2,500 might be the range I would use but your market may differ. 2.

The range of IRR's syndication/crowdfunding deals is much wider than just 15-18%.

Also pricing tends to be in the $250k+ range for SFHs, so cash flow may be difficult depending on how one is financing the deal.

If I had it to do again, I would press him for a range, but hindsight 20/20.