Which corresponds to some of your commercial loans.  

Or, they can demand a higher security.

Have others been able to secure a HELOC from their portfolio.

All 3 of us have great jobs and great credit so securing a mortgage shouldn’t be a problem.

from the documents a few BP folks sent to me a few years ago. it looked like they were investing in multi beneficiary mortgages IE multiple investors in a fractionalized mortgage.. which can be argued is a security .. in CA they are quite common but you have a specific disclosure document that must be filled out by a real estate broker .. it lets you know who the co investors are.. which is really the rub here.. none of these folks know each other.. and of course this is always the rub in a pooled syndicated investment that goes south.. 

Other than submetering, I'd recommend you ask to see the leases and whether each person has a security deposit that matches their present rent.

Because this deposit typically transfers over to the new tenant's security deposit, should I be charging an amount equal to the security deposit or only a fraction of that amount?

You are doing what most on this forum want to be doing and it appears by your correspondence that you have your ducks in a row and have planned for most of the problems you will face in the near future. 

Private owners are more generous with giving back security deposits, are less likely to raise rent, and free furniture is free furniture!

I would talk to a good securities attorney like @Jillian Sidoti to see what issue you may need to deal with.