I am not sure about GB games. The DS slot is capable of emulating NES games so I assume GB (normal or color) could be emulated as well.
You are correct about GBA though, Every GBA game has to be played through the Slot-2 (GBA Slot). Slot-1 (DS Slot) is not fast enough to send data to the DS so every GBA game that tried to run from Slot-1 (DS Slot) would ask for information faster than it can get it, and crash.
For Multiplayer (sharing) the best compatibility is the DS-Xtreme. The DS-X allows you to link with a flashed or non-flashed DS to a flashed or non-flashed DS. Most other Slot-1 carts require at least 1 if not both devices to be flashed, for many of the games.
For Multiplayer (WiFi) as I understand it, all will work fine regardless if flashed or not. I am not 100% sure on that though.
I had not done any reading on the SuperCard One yet, so I am glad you asked about it. It seems like a decent device. However, you need to link to a flashed DS, in order to get Multiplayer (Link) working. That isn't necessarily a huge problem cause you could use the SuperCard One as a flash device, you just need to flash every DS you ever want to let hook up to you. You also need to make sure you get as fast of Memory you can and preferably from Japan. I would personally go with the M3 Simply or R4 over the SuperCard One, though.
This post has been edited by throwingks: Jan 21 2007, 02:01 AM