Yes, I actually have screenshots of that too - there is a mod that allows you to play as the 2P counterparts in single player levels. You can even use Metal Sonic, Amy, Chaos 0 and Tikal in the Chao Garden.
THAT I'm not so sure about. Because they actually have the base game at $10, and then there's "Battle" DLC for $3. The Battle DLC gives you all multiplayer characters and levels, and Chao Karate. From what I've read, I THINK Big is in the base game, but once you get the Battle DLC it replaces him with the Dark Chao Walker.
If you do decide to get it, let me know - I can show you how to install those mods. Hell, add me on Steam too if you want - my profile is UnseatingKDawg - my profile picture is Mario and Sonic.