Hi,
Ok, I think you are overthinking this.
If you study the action "Pmt Spawn", you'll see that it expects the name of the pool and a reference to a prefab. So basically, your random pick of the prefab you want to spawn has to be done before you call this action.
I assume the following:
you have a "spawn pool" name "My Multi Pool" in your scene
in "My Multi Pool", you have created several "per prefab" pool, and each reference a different prefab
use "Select Random GameObject" and list the very same prefabs you used in "My Multi Pool"
Inject the gameobject randomly picked by "Select Random GameObject" action nad use that in "Pmt Spawn" referencing the pool "My Multi Pool".
that's it.
Does that makle sense? give it a go, if that doesn't work out, I'll do a working example. You don't need any custom actions, you don't need ArrayMaker. You would need arrayMaker if you want to have more control over the list of prefabs you may randomly pick.
Now. I can see a potential new custom action that would pick a random per prefab pool item declared in a "spawn pool", so that it would be more intuitive, but it's totally possible without as I explained above.
Bye,
Jean