"Will the scale bar be "at scale" ?"
Your scale bar's scale will be relative to an existing composer map item that is assigned to it:
QgsComposerScaleBar.setComposerMap(mymap_item_obj_here). So you will need to create the composer map item first, so it's scale can be referenced by the scale bar.
"I cannnot find anything from that page ScalBar class"
I found almost all the necessary functions to create the custom scale bar you want on that page.
Once the composer map reference is set, all other QgsComposerScaleBar attributes can be set using the class's public member functions, or those functions inherited by the scale bar's base class, QgsComposerItem, like setItemPosition().
Base class inheritance diagram
I guessing here, but for setting the opacity for the background to zero, you will probably want to set the QBrush used in QgsComposerScaleBar.setBrush(QBrush) to have a QColor with an alpha of 0.
EDIT (response to your further questions/clarifications):
Regarding the scale back groundI have been trying the following :
brush = ScaleBar.brush()
brush.setColor(QColor(255, 0, 0, 0))
I was incorrect in assuming the QBrush in question belonged to the encompassing frame. The brush is actually the color for the scale bar's font. The frame and its background are part of the inherited QgsComposerItem class. To effectively work with its methods of a similar name (setBrush() in this example), you need to access it via Python's super() function (using your ScaleBar object here):
scalebaritem = super(QgsComposerItem, ScaleBar)
This will reference the super class of the object, and set it to transparent black, much like setting parameters in the General options section for a composer item.
I could not change the font size !
You need to create a QFont, set its size, then set the scale bar's font to it. You can begin by pulling in the QFont from the scale bar.
f = ScaleBar.font()