I've owned my Strat since 1973, 6strngs_2hmbkrs, and I've played it every day since. I've had it set up many different ways, and for
slide, which is what the question is about, I wouldn't recommend going lower than 13s for the E string. I use 16s. If you want to continue playing the Strat for 'normal' playing and just use the slide from time to time, then you'll find them heavy, but you'll just build up some strength, which is good. If you stick with light gauge, you'll always be wondering how to get a good sound.
I've never had any problems with the bridge on the Strat. I screwed it down good and tight and kept the springs in the back ... It's as solid as a rock.
I believe Stevie Ray used 13s and up on his Strat. Heavier strings mean much more volume, but you've got to be able to handle them. They're not for everyone.