Well, If it was just general randomizing, you could use variables and set it to be random between 1~6 for each type: Fire, Grass, Water (Gen 1~6), and in another page, where it says conditional mark variable and whatever number of your choice.
Here is a example for fire starters:
Pg1:
@Control Variables: [XXXX: Random Fire] = Random No. (1...3)
Pg2:
-Conditions
Variable Random Fire: 1 or above
@Text: Would you like to choose Charmander?
@Show Choices: Yes, No
: When [Yes]
@Text: Here you are:
@Script: pbAddPokemon[:CHARMANDER,5]
@Control Self Switch: A = ON
: When [No]
@Text: Choose other.
Branch End
Pg3:
-Conditions
Variable Random Fire: 2 or above
@Text: Would you like to choose Cyndaquil?
@Show Choices: Yes, No
: When [Yes]
@Text: Here you are:
@Script: pbAddPokemon[:Cyndaquil,5]
@Control Self Switch: A = ON
: When [No]
@Text: Choose other.
Branch End
Pg4:
-Conditions
Variable Random Fire: 3 or above
@Text: Would you like to choose Torchic?
@Show Choices: Yes, No
: When [Yes]
@Text: Here you are:
@Script: pbAddPokemon[:TORCHIC,5]
@Control Self Switch: A = ON
: When [No]
@Text: Choose other.
Branch End
Pg5:
-Conditions
Self Switch A is ON
Blank Page
I made it in few minutes. This worked for me. Be sure to use the Choosing Starter switch to prevent the player geting more then 1 starter, and the Starter Choice variable if you need those.