Pete,
I've never done them under clear coat. I guess it would be possible as long as they dont react. My only concern would be, that stickers do tend to shrink over time esp if left in the sun. Dont know how this would go with the clear over them???
As for the detergent, you only need the slightest bit. I just used some washing up liquid in a spray bottle. First I cleaned the car, then wiped it with panel wipe. Spray a fine mist on the car and position the stickers. You will be able to move them a little due to the water but you should aim to get it right first time. Remember, the water mix is only there as insurance.. When they are in position, use a squeegee to push the water out from under them and any remaning bubles. A hairdryer and a small scalple can help if you have any pesky bits. Good luck.
Shaun