Sonic Adventure 2: Battle Message Board  

To make getting an all S Chao simpler, try breeding 2 chao that have A ranks in different areas until the A ranks duplicate nicely. You will probably have to say goodbye to some baby Chao during this process, but you will eventually have a Chao with 2-3 A ranks, making getting an all S chao easier.
To raise a grade, either give your Chao animals/drives that correspond to the stat you want to increase until the Chao has around 100 points in that stat (for Swim, Fly, Run, or Power) or just give the Chao any kind of fruit until it evolves (for Stamina) during the first part of its life (when it is a child Chao). When it evolves, one of its grades will have increased! Usually, I try to increase the lowest grade first because then during the Chao's life, that stat will be higher at level 99 than it otherwise would have been, making the points it starts with on the next life higher.
Also, it is possible to get a completely unhacked Tails Chao if you have Phantasy Star Online Episodes I and II. To get it, complete The Fake in Yellow and leave the area, but do not collect your reward. Instead return to the area and talk to the Rappy's until you find the one that is actually a Chao in disguise. You will then be able to download your Tails Chao from the machine that looks like a GBA. He will be named Tails and will have all A ranks. He is also a Chaos Chao. He cannot breed, but he cannot die

and for the people who would rather not buy PSO and get a GCN AR cheaper could always get a tails or other offical character chao codes from my FAQ.