Best thing that I have done is to charge a monthly pet fee and then put in lots on language in the lease stating that they have to have the ductwork and any carpets professionally cleaned at the end of the lease showing a receipt from a 3rd party explaining the next tenant may be allergic to pets.

My current goal is to look for properties for cash flow to help supplement retirement income in the near future (possibly 4-8 years from now.I could extend that time frame). 

I don't even consider them as supplemental income.

Are you taking supplements?

I'm open to getting a loan to supplement any extra income I may need.

I keep all of my reserves in a savings account earning 5.25%.Another consideration is how much steady W2 income that you have to supplement repairs and maintinence as issues arise.

This is just a preliminary filter and should not be supplemented for a thorough analysis of revenue, expenses, and possible expenses.

I would like to get more involved in real estate to the point where I can get 10+ plus units to help supplement retirement some day.I am looking to learn and open to any and all advice on next steps. 

And given the dearth of real talent at the hot corner these days (as evidenced by my fantasy baseball teams), I'd say you need to stick with baseball so you don't lose focus.Real estate investing is only for those people who are trying to get out of their day jobs and/or supplement their day job incomes to improve their quality of life. 

I would say I have a tenant who is seriously allergic and for his medical condition you simply can’t have animals in the building due to his condition