No. I have never used sheriff3 before.
I created my own skins. I have done many before without issue. I make my own in Photoshop and use OpenIV to add them to the models. Again, I have done this before, I have just never used slots others than default slots (police, police2, police3, sheriff, sheriff2, etc)
This is the gameconfig I am using. Is this the correct one?
https://www.gta5-mods.com/misc/gameconfig-for-addons-600