creative financing is going to be your best bet. each scenario is unique but no speicifc financing available to finance ground up, foundation and all ADU.Personal loans, LOCs, equity from primary residence are the top choices. 

I am looking for some recommendations of contractors that have done a basement foundation in Southern CA.

For the San Antonio market and I would imagine a few other in TX, it can vary, but to give you some ideaA cosmetic only flip, or a foundation and cosmetic only flip should be sub 1 month with ease.A full gut rehab with regular permits should be do-able in 12 weeks.A historic district rehab can stretch out to 9 months-1 year pretty easily due to the lengthy permit process.For something in the middle, probably the 1-2 month range.Each major item doesn't actually take that long but coordinating and pulling permits tend to make it take longer.

I put in about $40k in upgrading two of the units and a foundation repair and now it rents for $4,590/month.

That is, you are one bad roof, foundation, or sewer line away from losing your family member’s money, A.R. 100 years ago, I flipped relatively low dollar homes like this in Ohio.

What would you study (books, articles, blogs) in the next 12 months to form a strong foundation?

You haven't inspected the foundation, what if there are a ton of leaks throughout the house, mold, what condition the HVAC is in etc etc.

I also have friends that own one story properties with concrete slab foundations.

The foundation is key... lots of time spent in the basement looking at electrical boxes and heating units etc... the roof... 

The owner installed push piers because the foundation had settling issues but the tenant wasn't there when the push piers were installed and she said she got the unit that way.