can you mix crystals or use only one type at a time?

yes, you can mix crystals. Sometimes multiple crystals' healing properties compliment each other.

You can mix crystals but sometimes you will feel that certain crystals do not work well with certain others or they work better when complimented with another.

My feeling about this is that you can not have too many crystals in your environment.  After all, who is to say what your "environment" really is?  The crystals are all here existing in harmony here on earth (which is really just like one BIG environment).  So, since space is limitless, we are really surrounded by more crystals than we even know (what's the difference between having 20 stones in our bedroom and having millions on our planet?).  However, for some sensitive people, it may not be wise.  Each person is their own best judge for what is appropriate.  As loing as you feel good in your environment, lots of stones are not a problem.  I would only suggest removing some if you feel overwhelmed, anxious, confused, spaced out, etc.  It is all up to you!  Use your intuition to decide what is best for you.