So is a 30 year and just wait for rents to rise over the years to give me the cash flow I want for retirement a good idea...or is a fast pay down and cover the negative for a substantial rental income in the future a better idea?

Equity creates negative cash flow on a property.

You may see some effect due to age of credit history but probably not enough to take your score down to where you are seeing negative impact in your interest rate.As @Anton Ivanov correctly stated you will want to have a game plan for how to handle your DTI.

Vacancy at 5% of gross rent would equal $3,285 / yr, which ends at -$-473 cash flow.So if PM and Vacancy were incorporated at current rent rates, would lead to negative cash flow.What is RUBS?

Start pushing your limits too hard, go outside of the box too far, or abandon your core competency...this is the start of trouble.Having survived at least a couple of cycles I can say that chances are you'll feel pain when it happens but if you were disciplined when you bought your properties and stuck to what you are good at you can survive.In 2005 I regularly attended REIA meetings where people were bragging about the rental they just bought with $500/mo negative cash flow despite a neg am loan and nothing down. 

I have a seller that is current. He is upside down 30k on his home and wants out. Piti is 500,  market rent $1000. If I took it sub2, and rented it, would I be risking too much on the due on sale clause? If the bank d...

At these number you don't have good guaranteed exit strategies, and risk being stuck with a negative cash flow house if you don't want to live there.

If its negative, it adds to your debt.

So, I am still negative net worth, but it went less negative by right at about 40 percent.

You will have a negative cash flow and you will join the vast majority of newbies that fail unless you have a big pile of money to cover your monthly losses.Let's look at your suggested deal, $80K purchase price and $800 rent:Gross rent: $800Operating Expenses: $400NOI: $400Mortgage payment ($80K, 30 yr, 7%): $532Monthly cash flow: $132 LOSS (OUCH!!!)