I have a small investment project development in Boston currently on hold that I will certainly consider the partnering principal for.  

You will then have a few options with that rent money....you can put it into principal or you can start saving it to purchase yet another property.

The cool thing is that you decide what to do with that "house-hack money" you get from renting out the spare bedrooms....you can put it into the principal, save it, or invest it somewhere else.To every young person and/or newbie  purchasing property, I ALWAYS recommend to house-hack because this will greatly increase your chances of scaling and buying additional properties. 

When forming the LLC in FL is it possible to use my NY address as the principal place of business?

When you do it as a principal it’s neither for the seller nor for compensation. 

So yes, more expensive homes with lower rates can be different than cheaper homes in higher areas but then actual principal, interest, and insurance will likely do the off-setting. 

The principal paydown would be much larger, the appreciation and tax write offs would be better even with a weaker market because $1,000,000 down could buy a $4,000,000 property. 

You will have paid off some of the principal as well, but yes that ratio of how much of your payment is interest vs principal changes over the term of the mortgage. 

Basically if the interest, taxes and insurance (ITI) is equal to rents it qualifies instead of having to qualify rents against a full PITI (principal, Interest, taxes & Insurance).There are also some DSCR options that can use the STR like VRBO which tend to be much higher then the LTR - long term rents.

One avenue while you are waiting for that license is, getting a workman's comp (you could revise the principal's name later if you wish), and hiring him as piecework job, this would fall under Owner-Builder, this is only legal if 1.