Why shouldn't I mix commercial pet food and raw food?
Its best not to mix raw with any other type of dog food as their stomachs need to be acidic enough to break down raw bone. Commerical pet food is more alkaline and swells in the stomach. When the two are mixed it can upset the pH balance of the stomach and cause blockages when the bone can not be digested. This is at worst fatal and at best a expensive vet bill.
There is much misinformation about this as some dogs can "get away with" digesting both. So some owners will say, "I've been mixing for years and my dogs are so healthy!". They may have a dog with a particularly strong constitution but you don't know if their dog will have problems in the future.
I had a similar experience with a customer who fed their dog cheese every day, (dogs are lactose intolerant). They "got away with it" for years without any obvious issues until the dogs digestive system became so weakened that they had trouble digesting anything.
In our experience most dogs have much more stable stomachs and better health on a fully raw diet so why run the risk?
Though we don't recommend it if you still want to feed both make sure you put as much time between the commercial dog food and the raw food to allow the stomach acid to recover as much as possible.