It depends on what is meant by 'mix', and how long and complex a sentence is.Not in one sentence, but yes.
Consider:
Though rather long and somewhat purple, that sentence is using different tenses without breaking the rules of grammar.While he was shocked at the height of the castle's walls -- no-one back home had ever dreamt of building such massive structures -- what put the fear of the Gods in him was the impatient way the shadows on its surface moved this way and that, as if seeking an opportunity to break free, leap across the moat and tear him to shreds.
What one should do is to make sure that one's use of tenses is consistent and (where appropriate**) grammatically correct.
Had, Mouse, had....edit: Just spotted TJ and me have the exact same number of posts. *spooky music*
** - Dialogue is an entirely different matter. (And the first person narration of someone whose knowledge of grammar is not as wide or deep as it perhaps should be could require some deliberately (on the part of the author) dubious sentence construction for effect.)