TAILS VS SONIC
-Tails is yellow. Sonic is blue.
-Tails is a fox. Sonic is a hedgehog.
-Tails has two large fox-tails. Sonic does not.
-Tails can fly. Sonic cannot.
-Tails is a mechanical genius. Sonic is not.
-Both have different personalities.
-Tails lacks Sonic's speed.
-Both have small aesthetic differences that GREATLY differentiate them.
What does that say for Tails' playstyle? Slower than Sonic of course, but perhaps a higher jumping ability (similar to Falco and Fox). Possible jumping perks and recovery techniques can be applied to Tails, like gliding, multiple jumps, Peach's floating ability, or a superior Up-B. Perhaps he can use mechanical gadgets in his moveset. The two tails bring A LOT of potential to the table. VASTLY unique to Sonic.
I don't see what's so funny about it.
KNUCKLES VS SONIC
-Knuckles is red. Sonic is blue.
-Knuckles is an echidna. Sonic is a hedgehog.
-Knuckles has spikes on his knuckles. Sonic does not.
-Knuckles can glide and climb vertical surfaces. Sonic cannot.
-Knuckles is a treasure hunter. Sonic is not.
-Both have different personalities.
-Knuckles lacks Sonic's speed.
-Sonic lacks Knuckles strength.
-Both have aesthetic differences that GREATLY differentiate them.
What does that say for Knuckles' playstyle? Knuckles certainly wouldn't be as fast as Sonic, but he'd definitely be a lot stronger (think Marth Vs. Roy/Ike). His special moves and standard move would involve his spiked knuckles, which have tremendous potential. His ability to climb and glide differentaite him as well. Overall, he is pretty unique compared to Sonic.
SONIC VS SHADOW
-Shadow is black and red. Sonic is Blue.
-Shadow is more cynical, while Sonic is a happy, peace-loving go-getter.
-Shadow is.... virtually the same as Sonic in every single way.
What does that say for Shadow's possible playstyle? Absolutely nothing. Same attacks as Sonic, Same speed as Sonic, same jumping ability as Sonic... he's just a black Sonic.
Your logic is flawed.
1. Personalities don't mean jack in Smash. They do not affect gameplay whatsoever.
2. Aesthetics are also a minor point to nitpick and have no place in gameplay discussion.
3. What their profession is doesn't make a lick of difference either. It's not like Tails being a mechanical genius makes him the ideal choice. Nor does Knuckles being a treasure hunter. Also, glad to see you failed to mention that Shadow is an agent for G.U.N.
4. In terms of gameplay, you cannot deny that Tails and Knuckles are not similar to Sonic. They all run fast, it's just that Sonic happens to be faster. They all roll into balls. Hell, in the original Sonic games, the only thing Tails and Knuckles had different from Sonic was the fact Tails could fly and Knuckles could glide and climb walls.
They started as Sonic clones. Just like Shadow. And just like them, Shadow has evolved beyond that. You fail to acknowledge the powers Shadow has at his disposal, which are just as different from Sonic than Tails' tail attacks and Knuckles' digging. Or has Sonic this whole time been able to use Chaos Blast, Chaos Spear, and the like (other than plain Chaos Control. Sonic has been shown to do that, but has rarely ever done it compared to Shadow)? Point it to me, as clearly, Shadow only does what Sonic does.
5. You've completely botched Shadow's stats. Shadow is not as naturally fast as Sonic. He requires Chaos Control or his Air Skates to match Sonic's speed and/or sarpass him. Shadow is shown to be stronger than Sonic, being able to lift heavy objects over his head and overturn even heavier objects he cannot carry such as buses. Shadow has a whole repitoire of advanced Chaos Control abilities that Sonic has not been shown to use, such as Chaos Blast, Chaos Spear, Chaos Rift, Chaos Snap, Chaos Boost and the like.
While generally disliked by fans, Shadow is also one to use firearms, something Sonic has openly disapproved of. ("I wouldn't be caught DEAD with one of those!") While realistic firearms would never be in Smash, if Snake is an example, there are the weapons of the Black Arms Shadow can use that shoot energy blasts and lasers; typical alien firearms. Or rocket launchers; those are allowed.
Word of advice; play a Sonic game with Shadow that isn't Sonic Adventure 2 for once in your life, then tell me if they're exactly the same. Since clearly, you haven't played any Sonic game post SA2.
Want an accurate portrayal of how Shadow would fight? Go to 2:10 in this video and watch until the end.
http://www.youtube.com/watch?v=FHdNzU8IYGk&