People love tiny cabins in those type of markets. 

Yes when going for a personal full doc it will require all of that documentation .

Here is the city's page that outlines the RTLO, and has a link to the complete document: https://www.chicago.gov/city/en/depts/doh/provdrs/landlords/svcs/residential-landlord-and-tenant-ordinance.html. 

If memory serves they break down by quadrant and unit type.

It would be the underwriter that just needs to document the 6 month seasoning before closing in order to stay within the terms of Fannie/Freddie guidelines. 

Not only is this type of scenario conceivable, it happens (often to inexperienced investors who end up bringing their nightmare property stories to this very forum, asking for help!)

What property types am I interested in?

There is a second smaller window in the bedroom, that has the exact same layout (fixed center pane, horizontal sliders on the left and right).Does anyone have any idea for a window guard that would work or how to secure a regular window guard with these types of windows?

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Make sure you have all your documents scanned and ready to e-mail to them,  and ditto all your notes of conversations, etc.