I have a lot to say about Tobias's inclusion in the Animorphs, so bear with me. It isn't just that the Ellimist 'stacked the deck' by including him, it's thanks to the Ellimist that Tobias ever existed at all. He was born in a broken timeline that Elfangor used Ellimist technology to create. And, the Ellimist knew he would have to erase that timeline, yet, he waited for Elfangor to form a relationship with Loren, and for them to produce a child. The Ellimist then waited for the exact moment that Tobias came into being, and then erased everything about that timeline save Tobias himself. The Ellimist obviously had a lot invested in making sure Tobias existed. The question is, why?
The answer is, I think the Ellimist could see enough of the future to know that Tobias, as the son of an Andalite, might feel out of place in his own human body. Not consciously, but just enough of a subconscious inkling that he might not fully resist becoming trapped in a morph the way the others would. I think the Ellimist, on some level, knew that Tobias would most likely turn nothlit.
Fast forward to book #33 (spoilers). The Animorphs needed someone whose default form was neither human nor Andalite to fool the Anti-morphing ray (a technological advancement that the Ellimist could easily have predicted that the Yeerks would come up with at some point). If they'd sent a human, their biggest secret would have been revealed, and even if they'd sent Ax, he would have been infested and revealed everything anyway. One way or another, they NEEDED a nothlit on the team, or the series would have ended at #33.
This is also why, in #13 (spoilers), the Ellimist gives Tobias his powers back but doesn't make him human again. One reason, at least. I do also think that Tobias didn't really quite want to be human.
As for Rachel and Jake, the Ellimist says that their inclusion was a 'happy accident,' but I think that's bull. There's some crazy warrior-gene that runs in their bloodline. Jake and Rachel both found their calling in the war, Jake as a leader and Rachel as a warrior, in a way none of the others did (except maybe Tobias). That was no accident.
As for Marco, his mother being Visser One was what motivated him to fight, else he would have quit in #5. And he had a tactical mind that proved indispensable to the Animorphs, so it was important that he was motivated to fight.