You should read Steve's latest AAR about Nato and Soviets, it shows how active Sensors on missiles can go horribly wrong in a surprise attack.
As for scouting a Planet, try drones with passives.
They will show thermal and em-Sigs of planets, nearby ships, and if you want to check for hostiles, something will shoot at it.
Having a dedicated Launcher for that in a fleet isn#t too much of an investment, you can outfit scout ships with that equipment.
In a pinch, you can load it with a mirv for self-defense.
Or if you use large enough missiles, just use a standard launcher.
As for that task, your not using box launchers anyways, you could just switch out the ammo depending on the need.
Also, if you detect enemy emmisions on passives, you could fire missiles ahead of them, to a waypoint you suspect them to be at in 30 Minutes, and then light them up to confirm hostile short before arrival.