Hello guys, me again.

So I bought new pedals (shimano SPD-SL ultegra) a week ago and today I checked to see if the pedal had come loose and it had, so i tightened it, this time with more force.
My question is: Is it easy to overtighten pedals, I'm not using a pedal wrench, I'm using a standard 15 mm wrench and it is 19 cm long.
If you do overtighten the pedals can the threads in the crank arm strip? Or will the threads on the pedals strip first?