I'll base this off of everything that has already been said:
Since you've already decided that you need Bulbasaur, Rhyhorn, Slowpoke, and Spearow, we'll keep those as a constant and only fill in blanks for the other three.
Pick Vulpix over Ponyta. It has better stats. Don't add Nidoran, because it doesn't learn very good poison moves and you've already got Rhyhorn to cover ground moves. Magnemite is a good choice, but Raichu arguably outclasses it by having great speed (110), while Magneton will have a very low 70. It also has the additional fire weakness since it's an electric/steel type.
Edit: Nvm you got yourself set up already.
If you're just looking for a fly-er, just use your soon to be Dragonite for flying around (and even for that needed fighting type counter). That way you have an open spot to use an electric type, because your team needs that coverage.