When you enter an empty level and just start drawing, the default Level Type is Toonz Vector. You can change the default to Toonz Raster or Raster type if you want in preferences. You can also right-click on an empty level and select New Level (or just hit control-n) and then select the type of Level (Toonz Vector, Toonz Raster or, Raster) in the dialog box before drawing.
There is a difference between Toonz Raster and regular Raster levels. One big thing I've noticed is with colors (called Styles) you draw with. In Toonz Raster, if you draw with 1 style, you can tweek that color dynamically by modifying it in the Style Editor. In regular Raster levels, whatever color you put down, is it. You can't adjust it. You'd need to erase it and start over.
The smoothing of lines is a brush setting...in Toonz Vector/Toonz Raster levels it's called "Smooth". In Toonz Vector, might want to play with Accuracy as well.

Opacity of a color is controlled in the Style Editor when you've selected the color in your Level Palette. May also want to play around with Hardness brush setting in Toonz Raster/Raster levels.

I can't help you much with the new brushes as I've never loaded external brushes yet.
Hope this helps.