I don't have the ability to get that information right now, but it was developed by SquareEnix and only published by Nintendo. Since Mario, Bowser, Peach and Toad are licensed by Nintendo everything else logically is Square's property. Need go no further than M&L:SS for the GBA where Geno has a small cameo to see that.
Ugh! Well I hate to break it to you but back in the Melee days Chocobo received the highest votes for a third party character, even trumping Megaman and Sonic. As far as Slime goes, just a little fun fact, he was voted #4 in Famitsu's 100 greatest video game characters of all time. Since the whole concept of mascots seem to allude you I'm sure none of this is going to make sense.