Note: I know you said just Erika, but I figured I would include extra info because I would still say Vulpix-A for Erika anyways. Both of them definitely work though, and it may be easier for you to just use the Kantonian Vulpix you already have.
You should use Alolan Vulpix. It may not resist Erika's STAB, but it can hit quite hard. Ice/Fairy is a better attacking combo in LGPE than Fire is, being able to hit Giovanni, Bruno, Lance, and part of Trace's team super effectively, plus many random trainers around. It's also faster than many opponents with a 109 base Speed stat, which can mean the difference between life and death.
The only Steel type Pokemon in the game are Magnemite, Magneton, Meltan, and Melmetal. Of these, only Magnemite and Magneton are actually used by trainers, and they're not all that common. Besides, you have teammates for a reason.
Tl;dr While Ninetales-K certainly works well, Ninetales-A edges it out due to a superior attacking type combination and Speed.
Hope I helped!