Sure early on Bulbasaur makes dealing with everything up to Rock Tunnel a lot less annoying but you're giving up the best non legendary Fire type in the game (and is probably better than Moltres too) and Squirtle's Water Gun has far more uses on it over Bulbasaur's Vine Whip making it far more useful in Mt. Moon/Rock Tunnel.
Also Geodude, Onix, and Sandshrew are terrible against Lt.Serge himself as he likes to play like Guile and Sonic Boom everything in sight for fixed 20HP of damage. Which is inconveniently located at the one part of the game where fixed 20HP worth of damage is still potent (there's a good chance some of your guys are getting two shotted by Sonic Boom spam and higher defense pokes like Geodude and Onix tend to have less HP to make up for it).
Also also, Eggsecute and Eggscuator (or whatever the hell they're names are) are both Grass/Psychic. And also for Erika, why aren't you sporting Dugtrio? You can just grab one from Digletts' Cave and wreck everything forever with Dig, Slash, and eventually Earthquake.
I would object to Charizard being the best non-legendary fire mon. Arcanine is far and away superior - both in terms of typing and in terms of stats. But this hinges on what version you're playing.
Exeggutor has the advantage in Special attack over Venusaur but its low speed and defenses make it easily demolished. Also, the Bulbasaur line is the only halfway decent line that can learn useful support abilities like sleep powder or stun spore - most other mons that can either have bad typing (Venomoth, Butterfree) or bad stats (Vileplume, Victreebel).
I used to spend all my money at Cerulean City and then abuse the exp from Misty's Staryu by using only my Pikachu and defeating Staryu, leveling like crazy, then letting Starmie waste Pikachu, letting me start the process over without any penalties. A high level Bulbasaur/Pikachu-line duo handily took care of everything up until the Elite Four.