3rd Gen Filling out my team

Just looking for a little advice from advanced players: my current team is Charizard, Blastoise, Venusaur, (the latter two I traded for with a friend) Pikachu, and Alakazam. My question is, who should I put in that last slot? I was thinking Onix to get a rock/ground type, but I'm not sure, what do you guys think?
PS: I'm in Fire Red, that might change my selection

Since Onix can't evolve until you get in post-game, I wouldn't really recommend it. Since you're able to trade, Golem would probably be a better bet if you would like a Rock/Ground type and you won't have to wait til the post-game to evolve it. But if you don't mind the wait, Onix isn't really terrible, but it's not that great, either. Especially since there's not an Eviolite this generation.

You've definitely got your bases covered as far as Special types go, so going with a Pokemon like Onix isn't a bad idea. Like Ludger said, Golem (or Rhydon) might be a better choice than Onix if you're set on a Rock/Ground type, but if you don't mind overlapping the Poison type with Venusaur, Nidoqueen/Nidoking and Gengar could be solid members of your team as well.

Your team is really good Fire Red team, actually it makes me feel like an Yellow version. However, as you process longer in this game, I think you should replace Blastoise with a Lapras, using Ice element which can destroy Dragon type. And Dragonite can also be achieved. If you could trade with your friends, ask them with a good Machamp - great stats, great ability and Fighter is the type with counter most types in this game