@Ritch Bonisa thank you for the input, that's true I didnt put the codes into consideration, the house been empty for 5 plus years, who knows what's not in code, plus applying permit time around the area is a pain also, it could delay rehab time and increase holding costs. 

From 2009-2014, 2nd Home Mortgages were on the rise.

Another consideration is that if you are barely cash flowing on a home in an expensive market and you aren't taking into account capital expenditures, the first time you have a major maintenance repair (new roof, major plumbing or electrical issue, etc.) you will be in the negative for years to come.

This will squeeze the rental margins because the renters won't be able to afford more rent regardless of the cost of the rental SFR.So if my above analysis is correct - rental SFR inventory will be harder to acquire, rents will stagnate, SFR values will rise and cash flow margins will narrow.The moral of this analysis being that if you have a good "flipped" SFR, with low expected CapEx, that has a ARV less than $150K, that will rent for $1250/mn or more, in a suburban area that shows good appreciation potential ....

All contracts, options, or other devices not based upon a substantial consideration, or that are otherwise employed to permit an unlicensed person to sell, lease, or let real estate, the beneficial title to which has not, in good faith, passed to such party for a substantial consideration, are hereby declared void and ineffective in all cases, suits, or proceedings had or taken under this chapter; however, this section shall not apply to irrevocable gifts, to unconditional contracts to purchase, or to options based upon a substantial consideration actually paid and not subject to any agreements to return or right of return reserved.That appears to close the loophole. 

Also at my current day job, I make an about 35k per year so I'm hoping that the banks will take into consideration the potential rental income of the property when making a decision.

What considerations should I have with an 8-plex that differ from single family?

Take nothing off their rent, there is no reason to.You are unfortunately diminishing the potential buyer pool considerably by having a tenant in a SFH.

In Somerville, I think a 2/1 of any considerable size would rent above $2k per month (especially given its location).

My question for these investors is; have you evaluated what your cash flow will look like if your mortgage rates rise substantially?