I found these three "cheats" at cheatplanet.com :

Unlock Dragon Temple (From Fire Emblem)

To unlock the secret Dragon Temple game, play as either Marth or Roy. You must beat the All star mode on the hardest difficulty without
using a continue. (Or at least I didn't use one). Might work if u use a continue
This would actually be cool.

Unlock Sonic And Tails

To unlock sonic and tails, you must enter into multi man melee and choose cruel melee. pick your character and get ready to fight. Get a high score of 20 KO's and Sonic and tails will appear to challenge you at the SAME TIME. You must defeat them in order to use them. Good Luck. you'll need it.
This is most likely a rumor.

Get Koopa and Perin

To get Koopa beat adventure mode, classic, and allstar on very hard. ON the homerun contest hit on 1 char. 1300. Beat 10 man melee, 100 man melee, 3 min melee, 15 min melee, and kill a minimum of five guy on cruel melee. Do all 51 event matches, Do all break the targets and you must have mr game and watch to do this.

To get Perin beat adventure mode, classic mode, and allstar with koopa. Beat koopas break the targets and the home run contest. there may be more but im not sure
If someone could confirm these, that would be nice. I won't believe any of these until I have proof. (proof being your comfirmation )