You can wonder about mixing belief systems. When the Rambam combined Aristotle with Torah, that must have seemed at the time like mixing apples and oranges. However you would not ask that if one would apply that principle to Torah and Medicine. It seems like they deal with two different areas. But Aristotle's Metaphysics must have seemed to some people as not really a different area than Torah but rather a conflicting area.
It is hard to come up with a good rule of thumb about this.