Filter mobile devices MAC address

I am monitoring my network and I would like to know how many mobile devices are there. I am already filtering probe requests and matching MAC adresses with OUI identifier. However I have few doubts about this procedure:

1. 
   

   Around 90% of MAC addresses captured do not match any OUI. Can we assume that mobile devices will always have a known OUI?

   
   


2. 
   

   Is there any list with known OUI belonging to mobile devices? I would like to filter our laptops and PCs.

   
   


3. 
   

   I know it depends on many factors, but what is the average frequency more or less in which the probe requests are send?

   
   

Thank you!