How do you tell what weapon needs what ammo? Do all of a type of ammo (energy for example) go with the same type of weapon (like a 5 round powercell to a Driod Blaster)? Do some non-projectile weapons take ammo (like a Force Pike or Lightsaber)?
As for the type of ammo size there is an ammo field for the weapons. This should have a number on it (it below where it lists where you can equip it). This number is the size of ammo pack you need, and the damage type as Togan said, is the ammo type you need and is listed under the damage section of the weapon.
Because when you use it once, you lose the entire magazine. You can't use a 100-round twice on a 50-round weapon. Plus you have to factor in weight/vol. Why carry something the size of a 100-round if you can fit twice as many 50s in the same space?
The main advantage is if you're carrying weapons that take different ammo sizes, in which case you might find it beneficial to carry only the ammo size for one of them (either the larger cells, for convenience, or the smaller ones if it significantly increases the number you can carry in proportion to the larger).
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
^ Also why buy 100 round batches when they're more expensive anyways? If I'm outitting 120 NPC's and they all use a 5k round then that's a month of newbie salary after they shoot one full round out of their A295's. It might not be much but it would really start to add up in a long battle.
Why bother putting in a bulk order? I doubt it costs too much to produce. If I were you I'd put in a bulk order on something like explosives as once combat comes out those will probably be hard to get.
I guess I could produce my own ammo, but following that logic I could also do the same with explosives.
My goal is to have a whole bunch of everything I can get my hands on. Of course I have the priority of getting ships with sensors, and medical supplies right now but I also plan to have thousands of rounds of every type of powercell, and at least three of all the publicly available weapons (plus maybe some restricted tech) for bartering purposes as well as my own personal use.