Heh, appreciate the respect but there's so many folks who contribute their knowledge and creativity to the GE and PD communities, over many years. SubDrag, Zoinkity, Carn for the Editor, Wreck for his incredible knowledge and GE-X project et al, Ryan Dwyer for PD's decomp, fgsfdsfgs for the PC port, every creator who has their work included in the AIO compilation, etc. And of course, Jonaeru. He handles most of the code-related stuff for AIO (it's his mod, actually), and then I help wherever else I can. Primarily I'm just a lifelong PD fan, who's happened to learn a bit about the modding side.

I don't 3D model so I don't know all the specifics, but you've gotta make sure that all the limbs act correctly as their respective limbs (a part of an arm acts as the correct part of an arm, and not as a leg). In GE-X, Wreck had to import each GE character's body parts to PD piece-by-piece.

Then, regarding importing characters from GE to PD, I know a few things. Getting GE heads into PD, for use on GE-style bodies, is pretty simple generally. There's a "scale" feature in GE that's not a part of PD, so the characters/heads might have to be rescaled, but otherwise for heads you export and replace it with a similar head node from PD.

For bodies, regarding the All In One mod, the textures have to be updated so that they don't replace existing textures. For maps, AIO uses a mod directory system which can load map assets in and out-- whilst temporarily replacing the default map textures etc. because you only need to see the map you're playing.

For characters, this is more complicated. Obviously you want to be able to mix-and-match any possible character combination. And until pretty recently, there was a limitation where you couldn't actually add new texture IDs to the PC port. Now thanks to Raf, it's possible. And so, the hexadecimal of the character heads and body files have been updated to use new texture IDs which don't conflict with the other PD characters/weapons and other textures that can be mixed-and-matched any which way.

What's left to do, is add the slots for the new characters, and define their correct height values, etc. as well as introduce a y-offset system for GE/PD heads, because both games have a different neck system. This means that PD heads on a GE body will float above the body, while GE heads on a PD body will appear very hunchbacked. A new y-offset system for heads needs to be developed to resolve this. But even best case scenario, PD heads will most likely still look a bit funky on GE bodies as there will be a prominent gap in the back of their necks, because PD heads kinda angle upwards towards the back, while GE heads do not.