Re: salatul hajat
Alvena yes you can pray it for a need. You need to do neeyat of this namaaz n after the namaaz say the duaa given in the link and ask for your need.
Salat al Hajah - Salatul Hajat - Salaat Hajaat
Kinz.. DuaG is right. its upto you how much you can say. I would say it almost with every prayer, or specially with Fajar and Isha prayers.