I am interested in your statement " I think those were available in 4-8 and 16 ohms. So that could make a big difference on what you end up with. " Leaving aside from amp loading what difference to enclosure design would the driver nominal impedance make?
The impedance will not make a difference in the particular cabinet design-but rather the intended usage.
Let's say you need a tall cabinet to get the sidefills up in the air.
Something with a single 15 would not be tuned properly.Assuming a simple ported cabinet-but if you wanted to use an old JBL scoop design-that is a different matter.
But if you could use 2 or 3 woofers of a higher impedance and parallel them, then you might have a workable solution.
But if you have 4 ohm woofers, then putting 3 will present a problem to the amp-or even 2-if your sidefills are not stereo and you want ot run both cabinets off of the same amp.
THere are a lot of "particulars" that could easily change from one design to another.