Pokerade, Weezing works well for steels and ghosts, as it can learn Dark Pulse and Flamethrower. Heck, it'll even do nicely against slow psychics. One of the Pokemon I used throughout HeartGold was a Weezing, and it was one of my best Pokemon. As for Scizor, Technician/STAB boosted Bullet Punch will rip through anything that doesn't have good defenses, which includes quite a few psychics.
If you're still after a Dark-type, aside from evolving Eevee, you can catch Murkrow in the Safari Zone. I wouldn't recommend the Safari Zone, though. Bad experience. Shiny Ekans fled...
And with a Vulpix on your team, why do you say Steels will be a problem? You DO know they're Fire-types, right?