I do not know the laws in PA, but if there isn't a good reference to give, than a bad one would be the only alternative.
Yes they can. Just like a previous employer can give you a bad reference. That is why they have you fill out references so they can figure out if you will be a good honest tenant or not.
If the landlord feel that you was a bad tenant, he can give the landlord a bad reference.
"Just like a previous employer can give you a bad reference"...Where I am from that is illegal...A former employer can only verify that you did indeed work for them within a given time frame. I think it could be considered almost slander if an employer did that...