Yes, if you buy the entire set (it comes in a small box...with Axel Steel on the cover - in reference of On Tour.) then there will be a converter in it that allows people using the original Nintendo to play Guitar Hero. This works for both Guitar Hero: On Tour, and Guitar Hero: Decades. ;)
You could look it up, but there are, so far, three different Guitar Hero games for the Nintendo DS. Guitar Hero On Tour, Guitar Hero On tour Decades, and Guitar Hero Modern Hits. If you need more info on them, look it up.
Only in Multiplayer.
I don't think you can.
All of the Guitar Hero Games Except for On Tour, On tour Decades and On Tour Modern Hits
Some.
most likely
Yes, the guitar grip works on all DS versions.
no
I don't think you can. The DS can't transfer stuff like that.
They are two different games. Decades is a follow-up to On Tour. For example, GH 2 came as one to GH1.
Yes, inside of the guitar hero on tour box is an adapter for the first ds