New gun and holster, but the finish is already showing signs of wear. I want to put Neatsfoot foot oil on it. Will the pistol still be snug and secure or will the oil cause it to slip? Any advice?

I would say as long as you don't over apply it and wipe up any residual you should be fine. I've used neatsfoot on my ball gloves for years and never noticed any slickness after I wiped em off with a rag.

I soak my leather holsters with it when new before I ever put the pistol in it. let it sit a day or more to soak/dry and go use it.
helps the leather conform when worn and makes it water/sweat resistant.