If you really want to get into a lore discussion...
Simply put, shields would be layers of hot and cold plasma suspended in the vacuum around a ship. These layers would stop incoming shots and beams from impacting a ship. However, the magnetic fields suspending them are not impenetrable. If you get enough damage focused on one spot, the shield will become punctured. Once that happens, the defending ship will designate all shield power to that one location to attempt to close the hole. However, the way that they would do this is to increase the magnetic field holding that area of the shield up. However, magnetic fields are not enough. You get a big enough boom, and you will lose shield reaction mass. Essentially, that plasma is cannon fodder. You have to have it in the way of the shot or its going to do nothing so you use the magnetic fields elsewhere to push the reaction mass to the hole. However, this takes a few seconds. In the mean time, you have a few hundred ships drilling a hole through your ship by firing on the same location.
Nearly any bomb/missile in space would be using AM. The reason for this is that while there is no shockwave or fireball, you still get a massive burst of super-photons emanating from it which would shred shields. Now, an apple of AM risks ending life on earth. For this reason, the missiles wouldn't be large at all. And thus they have low mass.
Beams and lasers have no mass as they are shooting either photons or photinos. Either way, you still have zero mass heading towards your target. Also, it should be noted that since photons are singularities, they cannot impact each other, and thus, you can have an infinite number of beams overlapping without causing interference.
Also, targeting computers would not use traditional targeting software like we use now. Now, we use GPS, visual cues, thermal imaging and the like. They would most likely use magnotometers and gravitometers. The combination thereof would be able to identify where objects are in three dimensional space. The former would detect the magnetic fields emanating from any ship with shields and gravitometers (which already exist btw, they are just really really expensive and weigh 60 pounds) which would detect the mass of a ship. Since nothing you would throw at the enemy ship would have much mass at all, you would never interfere with gravitometers short of using the Egg's Gravity Bomb. Outside of that, nothing would come close to interfering, though more than likely, a gravity bomb would just make your shots more accurate because the target would have a massive gravitational entity following it.
At any rate, no number of ships you would ever have firing at the same spot would ever interfere with targeting. Also, you really think realistically that any ship could hold off 100 beams firing at it simultaneously? Good luck. The system currently is fairly realistic. My version would be moreso, but all the same I know what I'm talking about. Or, you could think of it this way.. The reaction mass needed to fire all those weapons is probably about 1% the mass of your ship. If you had a chunk of AM 1% the size of the earth, you wouldn't even be left with an asteroid field.