Well, Torchic is considered the best starter ever (that's why I didn't use it )
All of the other Pokémon (including the starters) are obtainable with an event (this could mean, for example, that you get them at a very low level - one of these Pokémon is obtained at level 5 when you are at around level 45, after a side-quest).
I personally think that these Pokémon are not easy to obtain because this way you can try out some other Pokémon you've never used before as an alternative, and you may even like them: for example, I used Jumpluff just for a gym battle, thinking I wouldn't use it much afterwards, and it's still being really useful, even though Whimsicott could be a better choice due to Prankster: I'm not that far in the game to get Whimsicott, so I'll keep Jumpluff, and if I keep finding it good, I may not even swap it for Whimsicott, even when I can get the latter!
Also, as other users said, you can't use the same team of six Pokémon during your entire playthrough, not only because "the more useful" Pokémon are available later, but also because a single team cannot handle everything in this game. You'll need to switch members often: for example, get a Dark-type to defeat a Psychic-type gym, then swap it with a Flying-type to defeat the Fighting-type gym, where the Dark-type Pokémon wouldn't have been as helpful.
Also, this seems to be your first post. You should introduce yourself to the other users (and me) with a dedicated topic in the Grand Hall section of the forum.
I hope you'll like Pokémon Reborn, and enjoy your stay in the forums too!