I played this game a LOT in my day (a couple years ago), specifically the Chao mode. I can tell you anything/everything about Chao.
Dark Chao
Raise a Chao with Dr Eggman, Shadow or Rouge. I would just pet the Chao over and over again and feed it and you notice it starts to get darker. Doing this also unlocks Dark Garden.
Note: When a Chao evolves into a Dark Chao, it's eyes turn into "angry eyes". I don't think there is a way to reverse this so if you don't want your Chao to have angry eyes then don't do it :P.
Hero Chao
Same as Dark Chao except with Sonic, Tails and Knuckles. You'll notice it's skin getting lighter while you do this. Doing this also unlocks Hero Garden.
Reincarnate Chao:
When a Chao reaches age 5, it will die. If you took very good care of the Chao during it's life, it will die and reincarnate as an egg. It will have the same name/personality, but it's skills wont be as high and of course it will be a baby.
Chaos Chao
If you manage to raise a Chao so well that it reincarnates at least 2 times, then guess what, you can turn it into a Chaos Chao. Note that Chaos Chao can't die and can't mate so once they evolve they are stuck like that forever. All you have to do is feed it 1 of every animal. You may wanna look up all the animals in google, make a list of them and tick off the ones you have found. When it goes to evolve, some funny music will play and you have a Chaos Chao.
Hero Chaos Chao
Repeat the steps for a Chaos Chao but raise the Chao with Sonic, Tails and Knuckles.
Dark Chaos Chao
Repeat the steps for a Chaos Chao but raise the Chao with Shadow, Rouge and Dr Eggman.
I know much more about them aswell. If you have anything else you wanna know just ask me. :)
Linear Mode
