One thing to keep in mind is there are programs that "Do Not" require employment seasoning so 6 months is not a factor with a few new programs. 

I would do the following, at a minimum:-request that he complete your rental application (hopefully you have a thorough application),-request identification documents (government-issued),-require that he provide you with proof of his visa status and the authorized length of stay,-verify his employment, income, and financial stability, and-contact his previous landlords for references.If he cannot satisfy these application requirements, do not approve.

6 properties over 6 years is impressive.If you continue on this approach, you may be able to do better than 1 property per year if you make more money with your employer and are generating cash-flow from your renttal properties.Why switch strategies if you have something working for you?

Trouble is that I don't yet have long-term employment in place (just moved to the area a month ago).

I have not had Voucher tenants before, I am thinking about asking double deposit as they are not making employment and salary criteria.

So I put years of work into renovating a couple run down buildings to provide apartments for people, did all the work myself and then was laid off my job.Since then I've been pursuing this dream of being self employed before its all over.

We tried to get info on the lender to ask directly but they wouldn't give it to us.I would have thought this would be a relatively quick process to do a credit check, check debt to income, employer verification, etc.current loan's interest rate is very low so they definitely haven't refinanced anytime recently.Additionally, having never assumed a loan before, anything I should keep an eye out for that might cause a hiccup? 

@Tanner Lewis W2 was more in reference to your full time income regardless of whether it is 1099, self employment income, etc. 

Also, no "company towns" like Flint, Michigan or Gary, Indiana where a single employer can pull up shop and the town no longer has an economic driver.

I manage large commercial and industrial properties now for my employer, I am a facilities manager for a large utility company in my area.