You know what 'poetic license' is? It's when poets (or writers) decide to bend the rules to suit their particular works. They'll make words that don't normally rhyme sound good together, or stretch the rules of grammar to suit their purpose.

Well, this is exaclty what musicians do, only with music theory. Normally, when you pick a scale to go over a set of chords, you'll want to make sure that all the notes of your scale fall somewhere in the chords, and vice-versa. That's the basic "rule" for having things sound good. The notes of your chords should be inside whatever scale you choose, or if you're going the other way, you should pick a scale that contains the notes of your chords.

This isn't to say that you couldn't play a chromatic run, or something like that, but most forms of music (blues, rock ,etc) are based around a particular sound and mood. So you should be careful when straying outside the rules, or you'll end up with a cacophony.