You can set your AMM launchers to launch X missile at every hostile missile, where X can be between 1 and 5. Personally, I start with setting it to launch 3 per hostile missile and adjust according to the intecept chances (Aurora handles the bookkeeping part, counting missiles from all ships engaging in PD duty)
So, yes, if the enemy is launching many small salvos, you would engage two salvos per reload cycle.
Stealth:
If you 100 size ship has stealth reduce its apparent size to, say 50, the res-100 sensor would detect it at 1/4 its maximum range.
If your ships size is _reduced_ to size 100, then yes, the res-100 sensor would see it at max. range.
Thermal is totally independant from actives and detects, well, thermal emissions (engines, basicly)
Your EM-tech influences efficiency of your actives, but the EM-sensor is as well fully independant from your active sensors.
As a rule, all of my combat ships carry a small thermal sensor as backup. My fleet scouts mount a huuuuuge thermal and a seriously sized EM (sometimes also huuuuuge). It is realy a _very_ bad idea to run around with a giant active sensor - active. It would be like a sub running around with its sonar pinging away all the time. Yes, you would see anything that enters sensor coverage, but every enemy EM-sensor will pick up your sensor´s emission from across the system.
I assume you meant "from 100m range", ´cause 100k is knife-fight range in aurora
re. missile sizes:
Steve tweaked it, so you no longer can make missiles < size 1. This was due to someone making missiles with dozens (hundreds?) of 0.01 sized chaff submunitions and the AI not being able to say: "Ok, there is no way those tiny things can carry any warhead, so I will only shoot at the bigger ones"