3rd Gen Good Idea or Not?

I Restarted my Pokemon FireRed.

Started off With Squirtle.
Caught Myself a Pikachu.
Caught Myself a Pidgey.
Caught Myself a Caterpie.

I Just Finished the First Gym. [Brock]
My Levels:
Squirtle: 13 [Level 8 When i entered The Gym]
Pikachu: 9, Trained it for Quite awhile. Going to use for the Second Gym.
Pidgey: 9, Trained it for Awhile, Going to use for Flying, and Fighting some Pokemon Types.
Caterpie - 4, Waiting till i get Exp Share, So then i can Get it into a Butterfree.

I have 2:10 in the Game already, Since there was times i put down the DS To do something else, And forgot the game running. Im going to be training these 3 Pokemon to around the Same level as I Progress in the Game, Then collect More pokemon and do the Same thing.
Is what I'm doing a Good idea for starting?

First Gym: Rock - Water Type - No Challenge - Finished.
Second Gym: Water - Thunder Type - No Challenge - Need To Complete.
Third Gym: Thunder - ???????? - ????????? - Need To Finish Second Gym to complete.

The Questionmarks for Third Gym is what my Question is.
What Type of Pokemon should I use.
and Would it be a Challenge?

I suggest Diglett for the third gym and the team help thread for further help with these decisions.

Le lock;