I manage to get 1K, like 2 years ago, but that was because I'm a Sonic hard core fan, and that's why I manage to find patience on it.
If you really want to get 1000/1000, I'll share with you my technique. Watch YouTube videos to see how to get S Ranks on stages. You can't die, on the exception of Crisis city with Sonic, I think. Anyway you'll naturally get the hang of it, learn the stages and from your mistakes. You'll dominate the stage in no time giving you the S Rank on every stage with every character.
There are some videos telling you the locations of the Silver Medals as well. Well that's all, good luck, and now that I think about it there are harder games to achive the 1000/1000, so don't let the game intimidate you
Ahh, one more thing, be really careful with Kingdom Valley with Sonic on both normal & hard
. This is the most difficult & gimmick stage in the whole Sonic history. Things get worse when you can't die, and even worse when if you die in the final running section that's so gimmick as well, you'll have to restart the stage all over