Hmmm, does the firing ship have to have ECCM to counter the enemy ECM or is it possible to have one ship with ECM and a active sensor lead the rest of the ship(s)?
Yes, the firing ship needs the ECCM, no the ECCM on the active sensor ship doesn't help.
ECM and active sensors don't interact. Regardless of how much ECM an enemy ship has, the active sensor will still detect it at the range the radar is made for.
Firing at the ship, that's when the ECM comes into play. If your Missile Fire Control (MFC) can get a firing solution out to 50m km normally, and the enemy ship has 10 ECM, then you might need to get to as close as 45m km (roughly) before you can fire.
Each ECCM works for the MFC its assigned to. If you have three MFCs, then you will probably want three ECCMs, one for each.
The 'cheap' way to get around ECM for missiles is to overengineer the MFC. If your active sensor reachs out to 50m km, and your missiles go out to 50m km, then you may want your MFC to reach out to 60 or 75m km. Then when the enemy ECM cuts your MFC range down, you still can fire.
With Beam Fire Controls, ECM affects your beam weapon's chance to hit. Again, you want an ECCM unit for each BFC. Over-engineering BFC doesn't help like it does with MFCs, so strongly consider ECCM if you are going with beam weapons.
Or can the fighter scout with the active sensor paint the target and then let the fighter squadron with only fire controls fire its missiles??
Ignoring ECM/ECCM for the moment, this works, in the sense that you only need one active sensor that can see a target before you can fire on it, and that sensor can be anywhere - on the firing ship, another ship in the same fleet, a ship in a different fleet on the other side of the system, on a PDC, anywhere, as long as it can detect the target.
The fighters will still need ECCM or overengineered MFCs to counter enemy ECM, or suffer the penalties.