Yes, they can be used for piercing your lip. The size is your preference but these piercing needles can be used for most body piercings. Only certain piercings require a curved needle. =)
A standard septum is 16g but I do not recommend piercing your own septum at home. You don't want to risk going through actual cartilage, an actual septum piercing only goes through a thin wall of skin called a "sweet spot".