I heard that if you move the personal mortgages to an LLC they could get called and we would want to avoid that3.

From my experience, taking it step-by-step has helped me avoid mistakes and build a solid foundation for the long term.

So $4,500 of additional costs to have a partnership only to avoid hypothetical risks would exceed the profits from many SFH. 

Otherwise, probably not as they know they can get out of a significant debt by running from and avoiding it and have shown they will do that. 

Avoid affordable (undesirable) properties that will typically have higher maintenance and vacancy, and buy A or B properties with less maintenance and better appreciation, which is where the real money is made.

Also- please note that most first time flippers need an experienced partner or trainer to avoid these pitfalls and to be successful. 

Avoid mixing relationships and business.

So I would try to avoid that kind of debt, and try to only get low interest debt if you are needing to borrow. 

You need to avoid certain high risk features, which include balloon payments within 5 years, excessive fees, validate that the borrower has the ability to repay the loan, make sure they are likely to repay the loan based on their credit history, excessive late fees, long loan terms, etc, etc. 

Since you're using an FHA 203k loan and house hacking, focus on managing renovations efficiently to avoid delays and unexpected costs.