My rental property is in Los Angeles county.

The loan approval is based on the property’s rental income covering the mortgage—making it easier for investors without traditional income streams to qualify.LLC Protection: Many DSCR lenders allow you to hold the property in an LLC.

My end goal is being able to flip/rehab properties. 

Depends on the specifics of the property.

Interesting article about “Squatters’ Rights” in California; how they originated and their intended purpose vs. how they are currently being taken advantage of.California's Squatter's Enable Tenancy Rights After Just 30 Days, Raising Concerns Among Property Owners About Housing Security

I have a rental home and another person wants to invest $100k and become tic and share in costs profits etc when I receive this money does the irs consider it a sale of the property and taxable or an investment and n’ont taxable?

It's common when you do these water payments that a few will pay the rent but not the water bill in protest.   

There’s likely a carrier that’s most aggressive in the city where your 4 unit is located and comparing your premium to that of similar properties  nearby will be the most accurate assessment and point you in the direction of the most aggressive carrier.

Selling a rental property too soon could mean missing out on a property boom.

Again starting out seems like the harder part - especially if I've never done it before . aside from my own townhouse (In CT)  - That's the only property I own and have equity in .