No penalty for natural spread, and penalty for missionary spread only impacts civs spreading another religion. Any civ without a religion will not mind missionary work though missionaries thhemselves lose faith power in foreign land. GP does not have that and can enter rival land.
With follower belief possible. i used religious texts enhancer so rarely had multiples mine rubbed others out. YOu can capture enemy great prophets, at least in town. i inherited a Great Prophet Christian when I took Moscow and accidentaly spread it there.