well, we can try inserting a machamp overworld, but I have no clue what it would look like. try modifing a sprite in overworld rebirth, and see if you can get one that looks good.
I can't do bulbasuar without inserting a new sprite, but there are some pairs witch might work.
lapras; big lapras
machop;machoke
spearow;fearow
slowpoke;slowbro
nidoran♂;nidorino
and clefairy;clefable.
those are what you have to chose from.
how about somewhere in the story a gang of pokemon attack you , like that episoe that there are alot of bulbasaurs and ivysaur and there one venusaur ,
like that they surround you and you have kill them all to get away but they should be alot weaker than you so its easy to kill them