I am under contract for my second duplex, and am thinking it will be time for me to begin hiring out lawn care, as I do not plan to have means to transport equipment between properties or dedicate time to a low value task.

Mortgage rates depending on credit scores are much lower.

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

I would use this report to get a credit and plan for maintenance items to come up sooner rather than later due to installation error on the new work. 

the Reason is if you do a sub too .. the buyer can default and it will take you  a few months to figure it out by then your have mortgage lates on your credit its hard to take the property back on the wrap you can easily foreclose etc.. really depends on what your credit score is worth to you .. ???

For now, I sort of want to start "small" and I'm looking to build a portfolio of single family homes in areas with good price-to-rent ratio where there's good cash flow and low price.

I have a complicated situation in showing proof of income, however, I have a great credit score.

I would not consider STR if you are not in the zone.The rent by the room probably yields you the highest revenue and lowest vacancy.Their fees sound way too low to me. 

CA taxes residents on worldwide income but may provide a credit for taxes paid to other states.

FICO as low as 640.