I know that the game can detect hacked Pokemon. Let's say I legitimately hack a Pokemon from PokeSav. Is PokeSav considered a hack for the game?
Yes PokeSav is a hack for the game. But note they have to be legit hacks. Yes they would be banned from wifi battles if they had like 9999999 EVs in speed. Like Sam4x said if it's possible ingame it's allowed in wifi battling.
You can use them as long as they have the right area where you got them,attacks,and stats.