GBA cable link
The GBA universal link cable is the only way to link two GBA's together, sadly.
Yes.
To connect a Game Boy Advance (GBA) link cable to a Nintendo DS, you need to use the DS's GBA slot, which is located on the bottom of the device. Simply insert the link cable's connector into the GBA slot, ensuring a secure fit. This allows for gameplay features that utilize the link cable, such as trading Pokémon between compatible games. Note that the Nintendo DS only supports GBA link cables with GBA or GBA-compatible games, not DS games.
eBay
you can't a GBA link cable can't connect to a DS.
There is a opening in the link cable for another link cable to be inserted into that which then can transfer to the other GBA's.
yes
Use a GBA link cable.
Yes. You must use the gba-gc link cable.
The GBA universal link cable is the standard in linking GBA's together, and it'd be best not to substitute any other cable.
Yes but you also can trade using a GBA to GBA link cable.