John if you're just wanting a Zoa-Tank honestly just about anything will work for them as long as:
A) You can maintain great water conditions
B) You give them appropriate light
C) A & B
I've seen tanks as small as 2 gallon with some pretty nice zoas in them but I'd think bigger is better. Zoas don't "fight" as much as other coral. I've got multiple types of zoas growing on the same rock. They sort of just grow around each other and in time inter-mingle as they spread. Some may be more aggressive growers than others and out-grow them for real estate but they don't usually engage in chemical war-fare.