Both the Warlock class and the Hunter class have many more Focuses with abillities that have not been revealed yet, so don't be pressured into choosing a class at the moment just because you've seen one or two Warlock Focuses and one Hunter focus. However, you are correct that every class can use every weapon. As for the levels question, Destiny is one cohesive world. Everything in cooperative mode will supposedly carry over to the competitive mode, and visa versa.