If Tifa and Aerith fought...

[Firo Volondé]

You're right, Aerith doesn't have exclusive capability to hear the planet - any living Cetra could. Hmmm... wait a minute! Aerith is the only living Cetra left! Therefore, Sephiroth was as worried about Aerith as he was about Holy, probably more, since Aerith was the one utilizing Holy, not vice versa.

Disagree. Holy was using Aerith to be cast, Aerith just went along with it.
Also, to be frank, the idea that the cetra, who are absolutely capable of looking exactly like and interbreeding with the rest of homo sapiens, died out is a silly notion.

They look identical to regular humans but possess a unique ability to commune with the planet and to guide the flow of its spiritual energy. All ordinary humans can trace their origins to the Cetra, having ceased to become Ancients when they gave up their migratory life-style and link to the earth in favor of permanent settlements and a more relaxed way of living.

Or rather, that Aerith is the only one capable of hearing the planet (which she canonically isn't, mind. Bugen comes to mind), if their abilities are genetic, seems incredibly unlikely.

Bugenhagen is not a Cetra. Otherwise he would have been imprisoned by Shinra, or certainly would have tried to use Holy, instead of giving Cloud and co. advice:

Bugenhagen: Holy... the ultimate White Magic. Magic that might stand against Meteor. Perhaps our last hope to save the planet from Meteor. If a soul seeking Holy reaches the planet, it will appear. Ho Ho Hoooo. Meteor, Weapon, everything will disappear. Perhaps, even ourselves.

Cloud: Even us!?

Bugenhagen: It is up to the planet to decide. What is best for the
planet. What is bad for the planet. All that is bad will disappear.
That is all. Ho Ho Hoooo. I wonder which we humans are?

so anyone can summon Holy once they have the materia for it? So even Barret or Cid can summon holy? did Mr. Gamescript ever say that?

Theoretically yes, though I'd give Cid a better chance than Barret. If a non cetra can use the horrendous kill 'em all weapon with just a little bit of study, it would only make sense that the override code could be activated similarly. Hell, I'd make it even EASIER.
Every other Materia in existence has absolutely no racial limiter regarding its usage, merely the knowledge of its application, and this includes Meteor. That Holy is the sole exception does not follow.

Being a major plot device, the White Materia cannot be equipped or used in battle, and technically does not exist as an item at all in the game engine.

Aerith: Only a survivor of the Cetra, like me, can do it.