The basic brushes in the Brushes menu are only adjustable using the standard settings for size, opacity and smoothness. You can change their automatic layering settings when you tap "Edit Brush" in the Brushes menu.

 

You can purchase pens, pencils and other brushes from the Brush Market and duplicate and edit these with the brush editor. The difference is that these pens are designed to work with the stamp-based brush editor. You can also create your own brushes following the steps in this tutorial.