Hey man... congrats on what I'm guessing is your first piranha!
I'm gonna go old school on yer ass here and ask a few routine questions.
Ya know when ya go to the doctor with a broken thumb, yet they still wanna know your height, weight, temperature, medical history, etc.? lol
A pic would be cool.... of the whole tank and maybe a closeup of him if that's possible.
What size is the tank?
What are you using in the way of filtration?
Do you know what your water parameters are? (temperature, pH, nitrate level, nitrite level, ammonia level)
More 'on topic': yes, it's extremely normal for him to be hiding.
First of all, he's a very young fish.
Secondly, he's new to this environment.
Thirdly, P. cariba is not exactly a 'schooling' fish, but they do live in groups, so he's prolly a lil' scared being by himself.
As for lighting schedule, I think it's best to never have the light be too bright, and have it on/off on a normal day/night schedule.