- People aren't loud in Japan. Generally males are quite shy or "cool"- they're quiet. Teenage girls upwards tend to be noisy with friends, but otherwise it's quite quiet.
- People are generally well-behaved in Japan (not counting the yakuza, of couse!)
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You'll notice I italicized the word "generally", because a lot of the items you mentioned are generalizations. They may be positive, or have positive intent, but they're generalizations nonetheless. So while I'm not opposed to the idea of Animorphs set in a different culture/time period/alternate universe I do want to caution you.
You say most people in Japan are very well behaved. This may or may not be true. Most people in the Boston area are very helpful and friendly to tourists, that doesn't mean you won't run into the occasional cashier who is out to rip you off.
How much of your observations are based on actually going to Japan or knowing someone from Japan and how much is based on what you've seen in movies, anime, Internet and books?