>>35620557>Earthbound>Mother 2>Gen 2>Kanto and Johto Pokémon only>PaulaWigglytuff - Can heal with Wish that can be learnt by Igglybuff, stats are focused on special attack (outside of HP, I guess), can learn a variety of elemental attacks.
>JeffMagneton - Can't think of any Pokémon that relies heavily on items, so Magneton because its a mechanical Pokémon.
>PooHitmonchan - Fighting type, learns a variety of elemental attacks.
Slowking could've also been a good Poo, since it can also learn a lot of moves of different elements, and is also Special focused with pretty good Physical stats. Slowking would also be resistant to Fire and Ice like Poo is, but I would expect Poo to be fighting type more than psychic, even though he's kind of both.
If we went beyond Gen II, Poo would definitely be Medicham, though. That just goes without saying. I think Klinklang would also be a better choice for Jeff in that case.
Anyway, Ness obviously only has three partners, so that's the whole team.
