Hey, metapod23;.
Well, I thought of a good idea.
We all know it's not possible to have Brock and Misty following you.
After you battle Brock, when the player continues towards Mt. Moon, you should make a script where Brock stops the player from going to Mt. Moon, saying he wants to join your adventure, and Misty will do the same. Just like in the anime, And they should walk ahead of you just like how your rival walks ahead of you.
To actually have Brock and Misty follow you, you should have them walk around in every town, make seem like they're actually with you.
And making scripts where they come to you and talk to you, after the conversation is done, they both should walk away.