We take their total members:
Level 9s : 1 (Empress(lvl9))
Level 8s : 7 (Maxim(lvl8), 6 miscs)
Level 7s : 8 (Alfia(lvl7), Zald(lvl7), 6 miscs)
Level 6s : 12 (Misc)
And add them together to keep in mind the 3 to 1 rule in the series, of which adventurers literally live and die by. For the sake of Pseudo levels, I'll count them as 2x the previous level: Pseudo lvl 13 = 2 Lvl 12s
So normal combat:
Empress = Lvl15
Maxim = Pseudo Lvl16 = 2 Lvl 15s
Alfia(no long chants) = Lvl 15
Alfia(long chants) = Lvl 16
Zald = Lvl 10 or up to the level of the enemy, temporarily
Other level 8s = Lvl 10 * 6 = 2 Lvl 11s
Other level 7s = Lvl 9 * 6 = 2 Lvl 10s
Other level 6s = Lvl 8 * 12 = Lvl 10 + Lvl 9
Total: Alfia + Empress + Maxim + Zald + Other 6-8s
Total(w/out UL chants): Lvl 15 + Lvl 15 + Pseudo Lvl 16 + Lvl 10 + 2 Lvl 11s + 3 Lvl 10s + Lvl 9
Total(w/out UL chants): 4 Lvl 15s + 2 Lvl 11s + 4 Lvl 10s + Lvl 9
Total(w/out UL chants): Lvl 16 + Lvl 15 + Lvl 12 + Lvl 10 + Lvl 9
Total(with UL chants): 2 Lvl 16 + Lvl 12 + Lvl 10 + Lvl 9
And then Zald can effectively reach close to the level of the enemy if the familia as a whole is able to damage it to the point he can eat some of it. So as long as the enemy they are fighting is level 17 he could reach high enough to end up being stronger than Maxim, and closer to the monster's power. This in a tentative manner puts the limits here at Lvl 16 for him… So let's change that out in the total power. Remove a level 10 and replace it with level 16
Total(w/out UL chants): 2 Lvl 16s + Lvl 15 + Lvl 12 + Lvl 9
Total(with UL chants): 3 Lvl 16s + Lvl 12 + Lvl 9 = Lvl 17 + Lvl 12 + Lvl 9
So, the total fighting capacity of the Z&H familia is Level 17 and some change… And that's against non dragon type enemies. Let's change it up with that dragon modifier…
So Dragon combat:
Empress = Lvl16
Maxim = Pseudo Lvl17 = 2 Lvl 16s
Alfia(no long chants) = Lvl 15
Alfia(long chants) = Lvl 16
Zald = Lvl 10 or up to the level of the enemy, temporarily
Other level 8s = Lvl 11 * 6 = 2 Lvl 12s
Other level 7s = Lvl 10 * 6 = 2 Lvl 11s
Other level 6s = Lvl 9 * 12 = 1 Lvl 11 + Lvl 10
Total: Alfia + Empress + Maxim + Zald + Other 6-8s
Total(w/out UL chants): Lvl 15 + Lvl 16 + 2 Lvl 16s + Lvl 10 + 2 Lvl 12s + 3 Lvl 11s + Lvl 10
Total(w/out UL chants): 3 Lvl 16s + Lvl 15 + Lvl 13 + 2 Lvl 10s
Total(w/out UL chants): Lvl 17 + Lvl 15 + Lvl 13 + 2 Lvl 10s
Total(with UL chants): Lvl 17 + Lvl 16 + Lvl 13 + 2 Lvl 10s
Now with the Zald modifier:
Total(w/out UL chants): 2 Lvl 17s + Lvl 15 + Lvl 13 + Lvl 10
Total(with UL chants): 2 Lvl 17s + Lvl 16 + Lvl 13 + Lvl 10
This completes their absolute maximum fighting potential as a collective. Keep in mind this really does come down to if everything goes their way. Otherwise their power is limited into their absolute strongest four.
Other notes: There are the obvious changes that would occur if say the ratio was 5 to 1, instead of 3 to 1. However, that is not relevant to this situation, as the scale already takes this into account by comparing characters against the average at x level when they have nothing but stats, so no fancy buffs here or there. As a result, each gap is exactly the same as it would be at lower levels, thus perpetuating the 1 to 3 ratio.