The Pokémon selection is somewhat random, yes.
Mainly, I wanted to avoid having Pokémon repeat between allies and enemies. Additionally, I wanted the playable Pokémon to be as distinct from each other as possible.
Regarding Generation 6, there are only a few Pokémon included because it’s difficult to find sprites for gens 5+. For example, Swanna has a sprite, but Ducklett doesn’t, so I couldn’t add it to the game.
The same goes for Pokémon like Crustle, Amoonguss, Carracosta, Simipour, and many others that could create interesting synergies but, unfortunately, I can’t include them in the game.
All the sprites are from PMDCOLLAB