Hi. Dresses and swimsuits (or anything with the "legs"-shaped bottom instead of a skirt) are adapted differently and the last picture shows you why.
So when you're making a dress, clone a similarly shaped dress (in the bottom area, mostly length-wise), transfer all data from it, then replace it in the same package. The severe distortion you're seeing are skirt bones that are not present in the package of the swimsuit but are present on your garment
- Clone yfBody_DressMaxi, export the blend under the name lod0, open;
- File - Save as under a different name (progress.blend);
- join all meshgroup into one and rename to Reference;
- append the meshgroup that contains the dress from your original blend file;
- you should never overwrite any data of the nude body, it will be messed up. I suggest to delete the arms and the neck for now;
- transfer weights, uv_1 and vertex paint from the reference. You do not need to edit uv_1 to prevent the skirt from being affected by leg morphs, the correct vertex color accomplishes that;
- expand the rig, make top visible and selectable, select it, duplicate it, delete all areas of the body covered by the dress, join with the dress;
- delete the reference, give the dress 0000, save the file;
- clone DressPanels, import the blend there, check how it behaves in cas;
- optional but recommended: if everything is fine, open lod0 again, append the edited dress there, split it the same way the original is split. Import in yfBody_DressMaxi;
- if you do not want to split, continue using Dresspanels but keep in mind that lod1 has 2 meshgroups. You'd just need to export it, replace one meshgroup with your dress and delete the portion of the fingers from it. No dresses affected by skirt bones are made of one meshgroup in all lods, so you cant use something else.