You are correct that PL's are not assigned by the FCC, however they may have been assigned by the frequency coordnator that the licensee went through. Though not legally bound (from what I remember) the applicant usually goes along with it.
I have found that most PL's out our way (Laxawacan?, Wayne, etc) seem to like 156.7.
An easy cheap way to find out is to find a scanner that has PL decode. Chances are, the PL devation at the receive sites may need to be checked. What has happened to me, after a PM check, the PL Defeat switch on one of our receivers was left on letting in distant stations.
Either way, unless the PL's are the same, your reciever should not be opening up on anything other than your selected PL (obviously).
Either way, if you are an authorized administrator of the system you are speaking of, a 10 min phone call should give you all the info you need.