My partner, who will co-sign the loan, will be living in the unit permanently and will also act as the property manager.

yeah still reeling from the notice placed on door by utility co notifying each of us water due to be shut off on Monday.  

I've asked and listing agent says, they have a contract with owner for 6% total, which they're offering co-op of 3%. 

In Colorado, the situation you described would probably end up with the listing broker changing his "status" to that of "transaction broker", where they handle the details of the transaction and provide all the needed information and disclosures but are not supposed to advocate for one party or the other.

What co-op are you offering to the buyers broker??

Would my agent need to "cosign" the lease in some way to make it valid since I'm not a realtor?

I'm a 25 year old University of Colorado graduate originally from South Florida.

As far as I know this is only eligible for primary residences and SFR, condo, PUD and co-ops. 

Over the years my husband and I have casually bought and sold a small number of investment properties and we currently co-own a 12-unit residential property in Utah.  

Matthew,It's either condo or co-op in New York City.