1: Not sure if they're humans but it's never been said that they disguise as humans while being on Earth so I would assume so, plus if they refuse to become a star guardian, they are free to live out a normal (I would assume human) life.
2: In magical strength, yes. In social strength, meaning some have more power in their sort of government than others, yes; This is proven by the fact that teams have leaders and veterans are respected for being veterans and not just treated like the rest.
3:Guardians so far have been shown to move and live together in teams of five with one of them being the leader (so far we have Lux and Ahri). It's unknown if all guardians are placed into teams of five or if they even need to be placed in a team, this is because Lux was originally the only Guardian on the rift. Aside from team leaders, the First Star seems to be their main leader, sort of like their version of God.
4:They were sort of asked to become star guardians, they could refuse and remain a normal human and live out a normal life, or vow their lives to protect the light. As far as bonding goes, I think they are given teammates based on the members' experience and power, from there they are sort of forced to become friends and work together.