Make sure they are swapped in the hlmt tag's dependencies and/or LoneID's. Check the same thing for the weap tag, and change anything that says 'mode - ...rifle/battle_rifle' change to your new model. (Don't forget there are some in the LoneID's, and not just dependencies)
I tried it making angel wings for my major mod, made the model and went to import and it said "input string was not in correct format" no one helped me on this either lol...
You can remodel the MC, but it will ruin all of his animations. Say you remodel the legs, when you run his legs will not move. They will be remodeled, but never move.