IIRC there are only about 6 full scale lauchers left over from the original station.
I sort of doubt there are any
launchers left over from the original Cardassian design, seeing how
1) the Cardassians supposedly thrashed everything worthwhile when they left, and
2) the launcher with six torps that Kira does have in "Emissary" is compatible with Starfleet torps.
More probably, the Enterprise
team just initially installed one Starfleet launcher in place of a destroyed Cardassian one as a nominal defense system, and gave Sisko six torpedoes to test-fire it with, and then left Sisko waiting for the next engineering team that would install more hardware (assuming Sisko didn't get booted out of Bajor before that, which was a likely scenario for Starfleet to concern).
The torpedos DS9 fires are mostly micro torpedos
We don't know that.
We do see in "Tribunal" that the station is stocked with warheads in big boxes that wouldn't easily fit inside "micro" weapons, but that of course tells us nothing about the size of the warheads themselves. (And curiously, the Maquis were supposed to have stolen the warheads while leaving the boxes behind, yet the Cardassians present the boxes
as evidence that O'Brien was the Maquis who stole them!)
We also see the pillbox launchers that fire some of the torpedoes (or compact glowing things at any rate) after popping out of the sides of the weapon sails, but we get no sense of scale. The overall scale of the station is vague anyway, and the "regular" launchers at the tips of the sails have no actual openings visible, so we can tell zip about the torpedo caliber that way.
The assumption of microtorpedo use thus doesn't seem to have any independent proof on its side, not even a hint in dialogue.