You can set the grid by clicking MIDI EDIT, then clicking inside the GRID box.
If you start with a low grid resolution, you won’t have much room to create polyrhythms. But you can always start with a low grid resolution, then increase it. When you increase the grid resolution, the beat won’t change, but you have more space to add more samples.
Tip: One way to swing beats is by increasing the grid resolution to 1/128th, then clicking inside the SHIFT box, and setting it to 1 grid cell. When you shift a sound 1 grid cell left or right, it will move 1/128th every time you shift it. This allows you to create lots of swing, all while staying on the grid. Once you set your Shift time, you can go back to the SEQ view to shift and tweak other parameters in real time.