Make sure you're locking in the armhelpers (stuff like prp_sleeve, bip_bicep, etc.) to the model as well. Also make sure you're using the Player model for scout, not the tf_movies HWM version. If it's still mangled at this point, try zeroing in the cosmetic as usual; then find what bone is causing the mangle; select it (and nothing else); and use the default slider on the procedural options and put it all the way to the right. If none of that helps then I dunno lmao

Ngl, I didn't expect someone to reply to this, seeing as how the sfm model is already kinda old.
I went back to the model and tried zeroing all the parts one by one to see what bone was causing the weird, and I realized that its the hlp_forearm bones that's messing up the model by zeroing them. If I zero all the bones except for hlp_forearm, and rotate and twist the hlp_forearm bones to make them look right, the model works correctly.
So thanks a lot for helping me realize what I did wrong here