Agreed that these are unique to both turrets. But the the clarification im looking for is in regard to the damage type the turrets deal and how it interacts with champions.

I went into a custom game to test this under a few conditions. The turrets(Shaco, Heimer, Zyra) deal basic attack damage based on the ap and skill level of the champion who created them. The point being that the damage source from the units is their basic attack. This is where ninja tabi is in question. Since the damage source from these units is their basic attack, this item should reduce their damage.

the damage from those units isn't a basic attack; it's spell damage. Think of it as an AoE spell that only hits one unit within the AoE at a time (like blitzcrank's static field passive), but these spells have a source that can be shut off.

They are not basic attacks. They are spells.

Heimer and Zyra deal mostly magical damage, so you can reduce their turrets' damage by getting magic resist items. As it is, there's no reason to get Ninja Tabi against either of them since they have some really annoying CC that Mercury's Treads counter them more effectively.