Will it work on the 3ds xl system with ds games

Yes. We have 2 3ds xl's. One the older one and one the new 2016 and now we apparently have unlimited cash on Animal Crossing New Leaf and unlimited poke balls.

Can you use it to play a ds game on the 3ds system

Yes, you can play any DS game on any DS system, including the 3DS. You cannot, however, play 3DS games or input codes for 3DS games.

would it work with dragon quest 9?

If it is a 3ds game it would be best to purchase Action Replay 3DS. More compatible games for Action Replay 3DS can be found on https://www.codejunkies.com/Products/PowerSaves-Pro__EF001113V.aspx