Dont if anyone asked before, but this is what you can try.
Make an event, and have those following ckecks
1) check if that pokemon is in the party. If pokemon isnt in the party, then push player away and tell them to bring that pokemon.
2) if player pass the first check, then ckeck if pokemon is in first (or other position). If not, push player away, then tell player to switch that pokemon to the position.
This is checking if you have bulbasur in the party.
pbHasSpecies?(:BULBASAUR)
This is checking if first (0 is first, 5 is sixth) pokemon is bulbasaur.
$Trainer.party[0].species==(:BULBASAUR)
Just put them in conditional branches.