I build and maintain Koi ponds, Waterfalls, and offer Lake management services as well.

Other than that, home has NOT been maintained and is in very poor condition.

The boilers are 50 years old, and I knew zilch about maintaining them, so I appreciated this. 

I like short term better myself because I don't have to deal with evictions, the guests pay in full before their stay so no chasing rent, and we get to get into the properties after each guest stay to make sure it's well maintained and issues fixed immediately.

However you wanna phrase it, you wouldn't be able to maintain profitability running a property management business with nothing more than a 10% monthly PM fee.

In doing so they would maintain their ownership in the property as being "investment" (and thus tax deferred) while it would still be considered primary residence for myself and thus qualifying for the home gain exclusion per section 121? 

This is assuming I can maintain break even or a surplus every month. 

HOA fees will eat right into your cash flow, you have no control of the HOA, and have no control on how well other people will want to maintain your building.Go look for a 2-4 unit. 

Are you responsible for maintaining common areas, mowing grass etc?

I think the danger in using percentages is that for less expensive properties, even when using a much higher percentage, the gross amount may not be enough to maintain a property.