small to small, and large to large, are both cross chaining. Any extreem angle of the chain is cross chaining. This will cause premature wear on the chain and chain rings, leading to early replacement.
If your front derailure is rubbing in cross chained situations, then thats why its rubbing, you are cross chained. if it rubs in other gear combinations then it needs to be adjusted. Or you need to learn how to trim the front derailure.

If you are in the small ring in the front. then the last 2 or 3 smallest cogs on the cassette in the rear would be cross chaining. If you are on the large ring in the front. Then the first 2 or 3 larger cogs in the rear would be cross chaining.

Trimming the front derailure means to move it a small amount to keep it from rubbing when the chain angle becomes enough to cause it to rub. Most modern shifting, on the front derailure, have a position in between each gear for this purpose. there will be a minor click in between the major click that signifies a complete gear shift. This minor click is the gear adjustment or trim position.