Short answer: No, you cannot mix tile sizes within a single map.
Long answer: There are two possibilities, but both come with their own hassles.
- If you set your map to 16x16 tiles, you can use the 32x32 tiles as “metatiles” consisting of 2x2 blocks of 16x16 tiles. This will effectively lock you out of many convenient tools, such as the terrain tool, so it might not be practical. If you’re placing all your tiles by hand anyway though, it might be worth trying.
- You could edit the rock tileset to be 32x32. You could either scale the existing tileset up and hand-edit it to not be pixelated, or rearrange the existing tiles and fill in the resulting gaps (for example, the horizontal/vertical edge tiles would get absorbed into corner tiles, so you’ll need to make new ones).
Personally, I’d ditch this tileset and draw/find another one that’s designed for 32x32, you’ll probably waste less time that way.