So, proper screening is EXTREMELY important!

The specialist I recommend you have is an attorney to properly draft the documents and a title company to facilitate the deal.

STR's, if done right, are viable long term investments meant to cash flow at a reasonable enough level to properly maintain and improve them. 

It’s so comprehensive that I’m compelled to give you a great answer, but there is so much to unpack that a full proper answer to every question may take longer than I have tonight.

But they don’t know proper thinning and trimming.  

When you analyze a market properly (1% Rules aren't properly), you're analyzing all the properties in that market at the same time.It's even more dangerous for newbies to use any Rule of thumb.Rules of thumb are nothing but shortcuts, which seems to be the growing method of doing anything now. 

Quote from @NA L Harrison: It is irresponsible of you to take in animals you can't properly house.

With a backup plan, and proper (extensive) legal docs, much of the risk can be mitigated.

I do have one question - I have a partner and we are looking into getting a properly together.

Most inspectors will know that properties only need to meet whatever the code was when they were built, not current code and they usually explain that properly IME.