Spring should be strawberry blonde (pink hair) and Autumn/Fall should be a ginger.  As for the Pokémon, a big factor would be the size of their parties.  Appearing to be field minions similar to the Hexagon Brothers (from [i]Pokémon XD[/i]) or lieutenants like the Shadow Triad (from the Gen-V games) their teams would probably consist of no more than four Pokémon; each one with a Sawsbuck headliner to fit the season theme.  With that in mind, my suggestions are as follows:[list]
[*]Spring would mainly specialise in Grass-type Pokémon with an affinity for Bug and Fairy types so their team would probably consist of Shiinotic, Accelgor, Sceptile and Sawsbuck (Spring Form).
[*]Summer would mainly go for Fire-type Pokémon so their team could consist of Salazzle, Centiskorch, Pyroar (female) and Sawsbuck (Summer Form).
[*]Autumn/Fall would specialise in Ghost-type Pokémon so their team could consist of Gengar, Decidueye, Dragapult and Sawsbuck (Autumn Form).
[*]Winter would obviously specialise in Ice-type Pokémon so their team could consist of Weavile, Glaceon, Alolan Ninetales and Sawsbuck (Winter Form).
[/list]