YES, THE DS AND THE DSI can play toegether.
no you can't play dsi games in ds but you can play ds games in dsi.
yes they are fully compatible (also with the dsi) (i've played together with a lite and a normal ds)
No, the R4 for the DS will not work on the DSi
No but DSi can play DS games.
Yes, the DSi can play DS games
Yes, the PictoChat can allow the Nintendo DS and DSi to chat together
they all play the same type dsi play ds games
Yes, you can play DS lite games on the DSi as well as the DSi XL
I know the difference between the DS and the DSi the DS you can play GameBoy Advance SP games and the DS games of course, the DSi ( though I don't have a DSi I have a DS ) you can not play GameBoy games on it you can also somehow download action replay cheats/codes which you can not do with the DS.
yes
Yes. Since Yoshi Island DS is a NDS game, the DSi can play it.
The original DS can play them, as can the DS lite. The DSi, DSi XL and the 3DS do not have a GBA slot.