Sunflower roots need constant moisture to thrive. Succulents actually prefer dry soil between waterings. Mixing them in a garden bed causes one type to suffer. It might look good in a temporary wedding arrangement. But for long term growth, their needs clash too much. I learned this the hard way with my own plants last summer.
My friend tried this combo in a big pot on her balcony. The sunflowers shot up and looked amazing for about three weeks, totally shading the little succulents. Then the whole thing just went soggy and yellow when she tried to keep up with the watering. She had to pull the sunflowers out to save the cacti.