The question about why they only use young girls and not adults to fight the aliens was never answered. Adults are so much more competent and experienced and able to deal with trauma then kids are, so why use children? Maybe there's a reason the aliens only land in schools. I think it's because young people adapt quicker so they are ready to fuse in a much shorter period than adults. Or because young people have minds still developing, but adults are more set in their ways, so the kid's minds are easiest to fuse with. I also think I know why they wouldn’t allow Yuri to quit the party. It's because there is a set time period for you to be able to fuse with your borg and if you switch to a different person then that messes it up. It takes about one school year for someone to be ready to fuse with their borg and they need new party members every year.

I'm pretty sure that the reason they do this is because the people who fuse need to be a certain age, and that if you're too young or too old there can be negative consequences for trying to fuse.

The reason for this if this is the case I believe is that they need to use a human that isn't done developing so that they can develop with the alien, but they can't use one that's too young because their body won't be able to handle it.