Multiscale Lugia is so far a dream radar exclusive.,. And dream radar pokemon can not be shiny.
Hacked Pokemon should not cause damage to your game. However, they are certainly less appealing to own, and not ethocal to use in competition. Or ethical to knowingly trade for that matter).
As for checking them...
Anything caught/bred in Kalos should be fine still. Pokemon caught in other regions and imported in through Pokebank you have too just look really close at. Look up if a pokemon can be shiny, what level it can be caught at, what moves it should know. If a Pokemon from gen 3 is flawless it should raise a red flag(fyi, Deoxys from gen 3 was the only time it was able to be shiny, hence why i asked about it being gen 3).... Gen iv only certain natures can be flawless if it was legendary or caught in the wild. Gen v legendaries, dream radar, and dream world pokemon can never be shiny. Oh, if it is at the level it is caught at in the wild, but is fully ev trained it is likely hacked. There are other things you can look for, but that is a lot of the major points.
