I know the Merged Tileset thing is already finished, but I'm really wondering why you need everything in one. RPG Maker XP has an unlimited vertical set, sure. That's great. But that doesn't mean it's an optimized play. I actually believe it takes a very long time to load maps that use a large tileset, and if you're planning to merge every one in the RTP, that's going to be quite annoying.
It's best to just stick it to some nice combination instead of merging EVERYTHING.