Can hero Elrond's ability to pay for allies of any sphere be used to play 0 cost allies like Dunedain Hunter if I have no Tactics hero? Does it count as a sphere match?

You cannot. Elrond has to actually contribute to paying for the ally. I know this has been ruled on before, but it isn't actually in the FAQ (I don't think, I just tried searching for "Elrond" there).

To note, if you reduce the cost of the next ally you play by 1 (with say Theoden or Grima) then Elrond also couldn't pay for that ally if the reduction resulted in a cost of zero.

This question has come up many times and the definitive answer is no. Good Harvest, which has a very similar ability, does not allow you to play 0 cost cards without resource match.

Thanks.

A more detailed explanation is that, if you read the rules carefully, you see that

1) playing cards with a resource cost requires paying resources from a hero of that sphere;

2) playing cards with no resource cost requires controlling a hero of that sphere.

In particular, 1) does not require controlling a hero of that sphere, so your Elrond/Good Harvest/Radagast/Treebeard effects work fine, because they're about using resources as if they came from a hero of the required sphere. But they are not OK for 2) because none of them actually give you the sphere in question.