is this combo all ok to use to flash my brand new ds lite? i've read many instructions on how to flash etc but they never mention which combos of flash cart/pass through device you can use to flash with - or is it unimportant?

 

It doesn´t matter.
All you need is a pass through device and a GBA flashcart.
If you want to save a few bucks, you can buy the cheapest pass through device or loan from a friend.
But the pass through device must be compatible with you flashcart!