Honestly you don't need anyone else. Pika can sweep Loreli with Thunderbolt/Thunder (and you can use Char for Jynx). Bruno will go down to Mew/Saur or Toise. Agatha will go down to Mew. Blastoise can sweep Lance (Pika can help for Gyarados). And Rival would take Toise/Physical Attack/Zard for the first three and then Toise, Pika, and a Ground move for the last 3 pokemon, depending on who he evolved eevee into. Honestly, if I were you, I would keep the 5 you have, and maybe even drop Venusaur. Blastoise and Pikachu cover everything he is super effective on.l
As far as moves go, I recommend:
Pika: Thunderbolt, Thunder, Filler, Filler
Blastoise: Ice Beam, Surf, Physical Move like Strength, Filler
Charizard: Flamethrower, Fly, Earthquake, Slash
Mew: Psychic, Filler, Filler, Filler
These are the only moves you need to beat the elite four.