Personally I wouldn't mix rice in with the raw meat etc. that I feed my dogs, but that's got more to do with me being a believer that dogs don't need rice or other grains in their diet than worrying too much about mixing raw and cooked food. That's not to say that my two never get grains because they have a bedtime biscuit most nights and sometimes a corner of toast.
Toast, don't mention toast. We recently discovered eze no only knows the word but knows it if you spell it letter by letter. She loves toast 😁
Having had Basset's and Clumbers most of my life I have found it so refreshing having Reese and not to constantly wipe up drool....thats unless I eat toast, its the only thing I ever see him drool over and guaranteed to leave a puddle on the floor!
I dont add any cooked food to raw but if you want to add things like rice, potatoes, legumes then these Should be well cooked if adding to raw food. Some of natures menu raw nuggets are mixed with brown rice.
I think the main thing is not to mix kibble with raw because its much drier and takes longer to digest, which can cause issues if raw food is in the digestive system too long.