The short answer is, yes, of course you can. You can mix travertine with a straw floor if you want, the key is going to be the transitions. You want to make sure you are using the same thickness of tile and a similar finish. Think of it this way, if you are trying to mix a 1/2″ thick tile with chiseled edges and a 3/8″ thick tile with a straight edge, its not going to work very well. The seam between tiles is going to be messy, and possibly sharp! Also, if one tile is thicker than the other, they can be laid together, but it is going to be much more difficult.
1. Can it be done? Yes.
2. What do I have to remember? Try and use tiles of similar finish and same thickness.