That is often a matter of opinion and will depend on the attitude of your municipality.Most municipalities do not require neighborhood approval, but will offer the public opportunities to provide input which will be considered.

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

All the leads I've found so far have balked at either the "investment property" requirement, the "in California" requirement (or both!)

They/you can't choose the carrier - their software assigns it.Fortegra is one of the carriers (paper in insurance parlance) that their product is written on.That said, Fortegra is A- rated I believe by AM Best so that isn't the issue.I'm going to take a guess and say that your umbrella is written with PersonalUmbrella.com (Markel) and they require all underlying insurance to be A-rated but also with an admitted carrier. 

The fix/flip might be a good way to get such a relationship started: usually has lower cash out of pocket requirements and you can sell the property faster to recoup the capital invested, plus any profits.

When we built up the triplex, we set it up so that each of the 11 rooms can be rented out individually (all BR, except 2, have their own bathroom behind an electronically locked door).

However, you can't sell something unstable, with circular references, requiring human override, etc.But, if you stabilized it, the model loses efficiency. 

Bought 2 SFRs on the same lot that are zoned commercial (found on Crexi) for less than the price of a normal SFR because of the value add required (like kitchens and full baths, lol), but should BRRRR nicely based on the LTR and STR projections. 

Then add any other codes required where you want to build and study those as well.

All too often I have observed individuals raid their retirement accounts, paying far too much in taxes.