Require Co-Signer on 3 Bed/3 Roommate

[Jen W.]

Hi! Here's my situation:

I have a returning tenant who left on great terms, will be renting out same unit he vacated and will have two roommates.  The other two roommates I do not have a previous relationship with.  Returning tenant and one of the roommates meets credit and income criteria, other roommate does not.  Roommate that does not meet criteria is a recent college grad with little credit and not enough income to support income/credit criteria at this time and offered his parents to co-sign.  All three people living in the unit would be on a written lease. 

My questions:

1. Do I require a co-signer for the roommate that does not meet the criteria?   

2. If I have a co-signer for this one roommate, do I do a credit check on the co-signer?

Thanks!