Yes, those are made for infants and are safe. Do you want to hear a crazy thing I saw! In the neighborhood we lived in with our first newborn, my neighbor had a baby two weeks before me. We had these houses where the garage was under the house so you had a stone wall alone the driveway. She set her infant carrier on that wall. It fell off . .. upside down WITH her newborn in it. Baby was FINE!! I got such a first hand eye opener as to how safe those things are!~!
You'll wanna get something to hold baby's head in place anyway so it isn't rolling side to side either. They have things you can buy but you can also use blankets and whatnot too. But make sure it is really snug around them so they don't accidentally suffocate on it.
Pretty much anytime, As long as it leans back enough so baby's head isn't falling forward before he or she can support it themselves.
You can use it as soon as your baby is born