We have sometimes seen our Pekin grower pullets fight in a cockerel like manner ..... seems to be just a growing stage.
Pekins are usually fairly easy to sex. We have some at the moment about the age of yours and very easy to tell the cockerels by their red combs and wattles .... large, well formed and bright red. We also find that most are crowing .... that funny strangled crow of a young cockerel .... by about 7 weeks.
Could you post some photos?