The Application Process

I've begun working with my supervisor and having him write my LOR for the MBA applications. I am looking at applying to 7 schools so that process is going to a long drawn out affair. I have a quick question regarding assessment and scope of some of the topics.

Duke asks Description of applicant's peer group

Is it ok to ask my Supervisor to assess my capability, performance compared to my peer group, or should he strictly stick to describing the peer group, its abilities and strength, capability etc. What is the scope to which he must limit his response?

I would also be interested to know whether schools accept a common LOR, given that he may not agree to write so much for so many schools (although i do have others lined up who do have a fairly good idea of my abilities )

Thanks in advance!