I like the advancement, but perhaps we could figure out how to do it without referring to "number of zombies"? I also think the way the HD range is laid out in the 2nd table is slightly confusing because the "base" HD is below the "starting" HD of the size. I also don't want to call the individual bodies "zombies."
To simplify a bit, could we make the merging process something that takes long enough it won't affect a battle immediately, so the DM doesn't need to track the corpses? Then we might be able to keep this framework but simplify somehow.