i did something similar by merging g2f & her pussy bit geograft.
here is an outdated & flawed account of the thing:
the thing is, when you Merge Geometries, the whole daz figure gets changed & will mess with every other future figure. Before you merge, you need to create a new figure that you can safely work on and that will be changed.
in anycase, even if you mess up, reinstalling your starter content should restaure your base figures.