Anything you can install via apt-get dist-install should not break your system.
Things you must downloa the .deb for should be fine, and are fixable. You should only run into trouble with source installs. Debian-based in particlar are quite robust in this way.
Even so - it is actually quite difficult to beak your system by installing software. Usually, conflicts or incompatabilities will only break the associated incompatability (I've known some things to disable sound for eg.) Unlike some systems where an "illegal" operation forces a reboot.
In general, best advice is to install the bare minimum you need for your purposes. Run only one type of app at a time if you installed several to try out. (It dosn't normally pose a problem to run more than one of a kind of app - it just eats resources. Where there may be a problem you'll find the system won't let one of them fire up.)
Most people break their systems by messing around with the settings, config files and so on. Epecially when they don't log their experiments. The most common thing to go is the X server - from one too many tweaks just to see what happens if...
Remember - linux is modular. You tend to break individual parts rather than the whiole thing.
Fool around and have fun. As always, back-up anything vital.