I can't see how a partner would hurt how much you qualify for as long as they have an income and better than horrible credit.  

Verifying last 2 years of rental history very important!

Make sure you have cash reserves for 6 months , steady w2 job if things go south and do not plan unnecessary big ticket items until you stabilizeI have known people start with 40 k and used it to fund flips and work their way up and get Business Line Of Credit for 200 K in 2 years and after that of course it is very easy

A mentor of mine with 400 units told me I can use a large seller credit to cover most of the down payment on a property I'm looking at.

Allowing them to move in depends on your screening criteria (criminal findings) and your history with the original tenant.

Your best course of action is going to be to really see what you can do to get your credit score up as quickly and as much as possible - you will likely be able to find options right now - but your terms are going to be much much better if you can get that credit score even up like 30-40 points

My credit score is 640 and DTI is 27%.

My current situation - I have a FICO of ~630 (no late payments or derogatory marks, just credit card debt from school). 

new to this and just looking for some advice guess i really i don't no where to start so any advice is appreciated bought my first house in may and thing seem to be going well so far its more of a boarding house / house hacking roommate situation with people i know for the moment plan on doing some things to up the equity but am limited on funds i know the neighbors house just sold for a bit more then i paid for mine so that's a plus and mine is nicer and in better shape i have almost no debt for the most part except the current mortgage on said house don't make much from my day job less then 3k a month after taxes and have good credit. i guess i just really need to know where to start should i be starting a LLC or something similar?

And I’ve done several 0% interest for a year credit cards.