With a $500k investment you could probably easily grow to a $1.65M, 6 units portfolio, with cash flow in excess of $2,500/mo, all while reducing your structure count from 6 to 2.

Electric Meters: I doubt the even if the ADU is a separate structure they have a separate electric meter as they were mainly used for guests.

3 of my faves when I was active for entity structure,  contracts and landlord-tenant have retired but Ryan Feeney of Overcast Law was very good with landlord-tenant matters. 

Structured correctly, entity paperwork in line, we're sure about credit and liquidity, we have the title and insurance contacts, we have pic IDs. 

What is your prepayment penalty length and structure?

Often when I work with real estate investors to create asset protection strategies, most tend to prefer a simple asset protection structure (something low-cost that does the job essentially).

We have a few hundred units in our pipeline and definitely looking to get some additional support within our structure.

Does this mean the mobile homes and or existing structures may be replaced on non-conforming properties also long as there are no additions made?

It depends on how the structure is written in the operating agreement, but under the structure I outlined above, that would be correct. 

Many of the refurbished structures seem to have new windows put in.