I actually know other investors from Singapore that got burned very badly in this market.

WE dont pay a fee for the credit line as I mentioned just 1/2 point for each new loan..

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Fico while they will look these are not credit score driven loans like those buying rental houses or houses to live in...

Just like most things in life there are good and bad to both choices. 

At the moment with 3 rentals (6 total units), I have one account and a credit card associated with it.

Whether bank or credit union - either will work. 

The homeowner collects a NET/NET 5% yield with 1% increases each year in long-term lease by a BBB+ credit rated tenant (similar to Morgan Stanley credit).

Also credit score 680 with no missed payments over the last 2 years and also check their criminal report just to make sure you know who these tenants are, as the eviction process in New York city is very long and expensive. 

Some of the best investment were made in bad neighborhoods and higher crime rates then as usual the area becomes gentrified and continues to grow in both equity and passive income.