I dont think flashcarts work for the XL DSi
-I use a cyclocDS(way better than most carts on the market)
DLDI(Dynamically Linked Device Interface) most recent flashcarts are already patched with DLDI so if u get a recent/good cart you dont even have to worry about that.
It doesnt ruin it, Ive used it and had no issues. the PPSE-DS runs on ur flashcart.
Yes you can edit any pokemon.
If u have a retail cart, you cant.
Well theres a way but you wont want to , you need a special adaptor and opening ur retail HG/SS game to solder some stuff, since these cartiges have different technology than past games.