Basic Question... do weapons listed on the base model of the ship already consume some of the hard point quantity? Example... the YT-1300 as in the EotE Core Rulebook lists it has 6 hardpoints... do the two Medium Laser Cannons take up two of those slots already?

No. Any hard points listed for a vehicle are "empty slots" that you can put attachments into.

However, it's worth noting that also means that if you decided to tear out a YT-1300's medium laser cannons that you don't suddenly gain two additional hard points to be used on whatever attachment you like. I'm not sure of what the official answer would be (or if the devs have even answered this sort of situation), but it's my understanding that when removing a weapon, you effectively have an open weapon slot that you can replace with a different weapon (such as swapping out the medium laser cannons for a quad laser cannon and a light ion cannon), but you couldn't remove one of the medium laser cannons and replace it with a non-weapon attachment.

Now, the first sentence of this post is generally true for weapons and armor as well, with the sole exception being lightsabers. With lightsabers, the non-hilt weapons (i.e. the ones that come with an Ilum kyber crystal already installed), 2 of that lightsaber's listed hard points are allocated to the crystal.