I've read the instructions and I proceeded to do what the addon was telling me to do for the NPC model replacement, however, when I spawned the Combine Soldiers with the model I had selected, they were T-posing and their animations wouldn't play.
I had tested with other playermodels and the same happened.

I also tried installing some animation fixing addons like that Universal Animation Fix addon, nevertheless, it didn't do anything about it and the NPCs remained T-posing.