Most piranhas will be skittish if you haven't been in their presence much before. Usually the more you're around the tank the less skittish he will become. I've been spendin too much time on this forum and not enough time in front of my tank until recently. Now my reds and my altuvei and rhom are less skittish as their used to me being there more often.
at 1'' i would think you could not tell what its going to turn out like .
i would just watch it grow spend loads of time with it to get used to you and see what happens
if you still unsure about it sell it and get something else