I think there no problem mixing diferent bait, i do it too but with a slighly diference... I don't mix diferents types of boilies.
My recipes takes some wheat, hemp, bird food, a bit of sweet corn, some pellets (various sizes) and boilies (a bit of crumble + chopped boilies), but just with one type of boilies. I do it to try the boilies, and always fish with the same boilies i baited.
In long sessions (24hr or more) I would mix more than one type of boilies just to try to see what boilies work better in that water. Add chopped boiles and a bit of crumble of 2 or 3 different boilies to the mix, bait an area and cast to there, trying one diferent boilie on one different rod.