I mix pureed fruit in DS's oatmeal all the time. He likes it. I introduce the fruit on its own first for 2-3 meals and then will start mixing. We have done apples and prunes so far.
You absolutely can but my personal preference is not to do that until you've introduced the puree by itself. The cereal changes the flavor and consistency so it doesn't give the baby the true experience. But there's no harm in mixing with cere
Re: Solids question
Ditto the PP on introducing the puree alone first, but looks like you are already doing that.
I only add breastmilk if needed after mixing the oatmeal and frui