The early and the later game parts are not compatible because of the different i-patches. Also the replay gallerys would be overwritten and only the latest one then available. But actually the included save transfers between the parts with the game status check would make a combined game not working anyway.
Yes.. I mean, i didn't play this game for a while even though i saw so many times since the name and pics seemed like an average harem game(which kinda is) but it is fun imho