This is my first time replacing hair mesh. Last week i realized i would need front bangs on Anto-Youth hair mesh, and decided to get them from Anto-Ines mesh, since they share same textures (for private use only, of course).
I followed this hair making tutorial https://modthesims.info/showthread.php?t=622787 with prior knowledge of linking and extracting steps from my accessory projects. After i collected mesh files and extracted them from Milkshape as .obj, i started working on them in Maya. First difference i noticed is that extracted Anto-Youth mesh had 3 differently named groups: alpha5, alpha6 and alpha7, instead bangs_l1, bangs_l3 and bangs_l5, despite project being based of Anto-Youth (and using same sims base mesh). I decided to keep them as is.

When i was satisfied with new mesh and new UVs/textures, i exported it as .obj and imported back in Milkshape. I replaced old groups with new ones, and first thing i noticed is difference in hair position and size relative to Milkshape skeleton. For some reason after being exported to Maya, mesh got bigger and changed position along Y-axis. Between .obj export from Milkshape and .obj import to Maya, something changed. I decided to keep testing regardless.
I assigned all fixed weighted vertex to head joint and proceeded to export to Blender for painting weights, as 5gd. Blender 2.9 can't import armature from Milkshape.
So, i switch to this relatively newer weights tutorial https://youtu.be/wL2v7MjaM3M Sims Base Models with Armature provided in tutorial are still from older Blender, and Armature just doesn't get assigned to hair groups no matter what, thus preventing me from selecting bones to paint weights to.
Furthermore, 5gd exported from Milkshape gets imported in Blender as rotated 180 by Z-axis, and much lower then Blender armature.
I go back to Milkshape and export original Anto-Youth Mesh groups for MeshToolkit via TSRW, and try to copy weights that way. Clearly there is still height and size difference in models that shouldn't be there, as they don't match perfectly despite only front bangs difference.

Not a single step between Milkshape/Maya and Blender makes any sense, and is honestly driving me insane. Maya step specifically, because i work in game industry as modeler and i have no idea how exporting .obj from Milkshape to Maya and then back to Milkshape changed it's size and absolute/relative? position.
Any help is much appreciated!

Small tip for working with hairs is to also extract the back of the head and the face (age/gender prefix +face or +hairbald, so afface/amhairbald/puhairbald/etc.. Extract the faces as OBJs, and the bald heads as GMDCs or the format that works in Blender - OBJ also works). This lets you know a bit easier if the hair actually shifts or changes size, maybe especially if you use the obj versions, since those aren't linked to the skeleton.

I'll let someone else handle the Blender parts that has to do with the rig/bone assignments. If you need any tips in Milkshape, though...

The rotation issue could be a problem with how the axis system is set up in Blender (I don't think it can be changed in Milkshape?), or how the export works with this system. If there are any options, make sure to check this before exporting out from Blender. You can test with an OBJ mesh forth and back to see if it's an issue with the rig or with the program setup. Same with Maya (an OBJ isn't affected by rig issues, so if everything in the program is okay, it shouldn't change position or size. If it acts up, it's likely the program or export/import options. If it's fine, it's likely something to do with the rig)