The other accpeted translation "I am not tired for nothing" means that the speaker is tired, while "I am not tired at all" means the speaker is not, they are opposites!
Which is the correct meaning? One of them should certainly be taken out.
"Per niente" is almost always "at all" in negative sentences; "for nothing" I would translate with "per un niente", "per un nonnulla", but that's minimizing rather than "nothing".
Actually, "I am not tired for nothing" means more like, "I am not tired for no reason" -i.e., there is a reason i am tired.
if the speaker is feminine do you say " stanca" ? and "stanci" for plural masculine?? etc
In English two negatives make a positive. Therefore I am not tired for anything is correct!