Can each round of my tournament be a different length?

Yes, in a knockout. You might want quick early rounds and a longer final. A league uses one length for every match.

1. Open your tournament and go to Settings.
2. Under Match rules, each round has its own row: Round 1, Round 2 and so on, then the Final.
3. Tap the race you want on each row. It saves as you tap.

A race to 2 means the first player to win two frames wins the match.

Change it before the draw. A match's length is set when that match is drawn, so changing a row afterwards doesn't change matches already drawn.

If fewer people turn up than you planned for, Round 1 is still the first round played, however many there are. In a 16-player knockout with 8 players, the quarter-finals use Round 1 and the semi-finals use Round 2. The final always uses the Final row.

See how many frames should each match be?