I have run a mixed size multiwire circuit before. I see no problem with it.
We remodeled a small kitchen in a office building. They had a removed a stove fed with 6/3 AC cable.
I used one side of the 6/3 to feed a 30 amp 120 volt water heater and the other side to feed 120 volt 20 amp outlets.
No worries about voltage drop.