This situation can happen in Single and Double Elimination brackets, when the number of the participants is not a power of two (2, 4, 8, 16, 32, 64, etc), because they cannot be halved until only one participant remains. Those who skip the first round means that they receive a "bye" to make the bracket work.
Read more about "byes" here.
You can do that by pressing the "Reset bracket" button, situated in line with "Edit brackets results" button.
In “Edit match score” popup, just press the “Add new set” button to add more stes, up to 10 sets per match.
Yes, the bracket is not mandatory in tournament creation page. You can add your participants and generate the bracket whenever you want.