Is it possible to loosen or crack the tire when I change a broken tube?
Yes, you can definitely damage the rim and tyre. But normally you just pinch the tube. The little dents you make have sharp edges which are not good for the tyre, and you might possibly poke your tool through the sidewall, but it's extremely unlikely that you will break the bead of the tyre. You might even crack the rim by excerting too much force (this is normally a problem for people who resort to metal tools after breaking plastic tyre levers). But if you're using that much force, you are doing it wrong.

Tyre levers are cheap, buy some. Michelin (almost flat, yellow rectangles) or Schwalbe (blue ovals) are the better brands IMO.

The "magic" technique bike mechanics use for removing tyres is to pick one point on the rim, pull the tyre out at that point, then run your other hand round pushing the bead into the slight depression in the middle of the rim. That gives you extra slack at the hand that's pulling out, making it easier to get the tyre off. It also makes sure the rim isn't stuck to the tyre at any point, and makes sure the bead is disengaged from the hook of the rim. I usually use a tyre lever for that so that any rough spots or objects embedded in the tyre don't end up in my hand. 

Look at a rim cross section to see what I mean:



Two things to look at: the hooks that "grab" the bead to hold the tyre in place, and the slight curve that creates a low spot for the bead to sit in while you're getting the tyre off. Done this way I often don't need tyre levers except for running one round the rim to loosen the tyre.