They can coexist. Give the acan room to open up at night and grow. I have not seen it, but I have heard that zoas will lose if touched by the acan feeding tentacles...for lack of a better word. I have both in an 8 g but have provided lots of room.
I gave them room I thought....but the zoa's keep creeping closer. They grow faster than I thought. I don't mind losing some zoa's to the acan. Just as long as the acans can fight them of I can live with it.
Hope it will make a very colorful field together in the future