The odds of cracking the frames is pretty low, & it's not something I've ever heard of, even in all pastic ports
. Though going loco when tightening them will make it so that you'll need to replace the gaskets sooner, due to excessive compression
causing permanent dents in them.
I'm not sure what you have for levers or knobs. But as a guide, if you have wingnuts, or knobs of a similar diameter, you'll want to stop tightening things a little before there's discomfort from the pressure on your fingers. And if the levers are longer, then use proportionally less force.
The threads are doing most of the clamping work, & don't really need much help from you.
Also, you can start with them a bit loose, & tighten things up a bit if you're seeing small leaks
. Which is something that you can test for anytime, via a bucket full of water
or a hose. Just put a towel down first to catch any drips.