Fjonis, it is definately possible to have more than one mod. You installed DyP to your .../Civilization III/Scenarios/ folder, right? And then you go to your ../Civilization III/Art/Terrain/ folder, and backup all those files (copy and paste somewhere safe). Then, when you download Rhye's vanilla terrain, you put it in ../Civilization III/Art/Terrain and overwrite all the files there. Is that exactly what you did?
PS: When you have DyP, you can still play normal games without DyP, you know that, right?