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?