How many hard points does it take to add additional turrets to a ship as a modification? In reading the ship and modification sections, I have not found reference to fire arcs at all. If his has been covered before, I apologize. I tried search and found very little relevant.

Mounting is part of the mod purchase cost. It's not mentioned as a specific thing because typically it's not worth the effort since weapon fire arcs rarely matter in vehicle combat in this system.

So in this system a TIE can mount a quad medium laser turret without issue?

In the upgrade section under weapons it clearly says that a single weapon takes up 1 hard point, and if you add 1 or more weapons to give it Linked X(where X is the number of secondary weapons you are adding) it takes 1 more hard point.

As for Fire Arcs, its really whatever you say you are mounting it as.

And if you are replacing an exisiting weapon system you get to recoup HPs. IE: swapping a twin-medium laser cannon will net you 2 hard points(1 for the base system and 1 for it being multiple weapons on the same mount) and you then add the new weapon system. So swapping a linked 1 medium cannon for a single Turbo-laser will actually mean your ship has 1 additional HP.

So in this system a TIE can mount a quad medium laser turret without issue?

It can mount a turreted weapon, yes. But a Quadlaser has a minimum Sil that the TIE doesn't meet, and 4 Linked Laser Cannons has the noted HP issue (which isn't worded well, so maybe it can depending on how you interpret the rules.... even the Dev responses on that weren't totally clear.)

The thing is mounting a turreted weapon on a one man craft like a TIE fighter doesn't really matter in this system. Fighters are typically allowed to shoot at whatever they want because they are small maneuverable fighter craft and that's what they a were designed to do, with the limiting factor being that as a one-man fighter they'll only be able to shoot at one thing per turn anyway unless the pilots got a special talent or something that allows him to shoot more then one weapon. It matters a little more when you start talking multi-crew fighters like the Y-wing because it allows the ion gunner to shoot at a different target then the pilot's laser cannons. It matters most on large capital ships which aren't maneuverable enough shoot whatever direction they want and so keeping track of what target is in which arc and maneuvering to manipulate that is more relevant.

How many hard points does it take to add additional turrets to a ship as a modification?

To answer your specific question, there are no rules (as far as I know) regarding which weapons have fixed firing arcs and which are turrets.

When you add a new weapon to a ship, there are no rules as to what the firing arc of the new weapon will be or whether it will be a swiveling turret. It's basically up to you to decide; and deciding to make the weapon a turret is (as far as I know) a free choice (since no rule says it costs anything).

So why wouldn't you just choose to make every weapon a turret? Well, officially there's no rule that says you can't ; but I suppose the designers are counting on people to exercise discretion and act within the bounds of verisimilitude (e.g. no 360-degree turrets on a TIE fighter)

However it does stand to reason that there should be some mechanical limitation on this. I would suggest ruling that mounting a weapon system as a turret (i.e. giving it "Fire Arc All") requires 1 additional Hard Point and adds, say, an extra 25% to the cost of the weapon system. Those who won't (or can't) pay these extra costs must mount the weapon with a fixed firing arc, chosen at the time of installation.

I think HP are too rare in most cases to make adding an HP to the cost to make a weapon a turret justified IMO. Also what would you do when modding a one arc weapon into a two or three arc weapon, o a two arc weapon into a three arc?

Easy. You remove the one arc weapon(freeing up hard points) and then you buy a new/use the old weapon, and make a turret with the desired arcs.

As for cost, the only thing to do is let the GM decide.

Adding Linked weapons doesn't take more hard points than adding non-Linked weapons. The difference is only seen when replacing weapons.