If you only want customised behaviour for a couple of columns (or rows) then you only apply a custom delegate to those columns (or rows). This is exactly what your own linked combo box delegate does.

Jthomps approach is also a way to achieve the goal. The paint() and createEditor() functions look at which column they are being asked to act on and modify their behaviour accordingly. This method ties the delegate fairly closely to this particular view, i.e. You cannot reuse this delegate class on another view with 8 columns to get customised behaviour on columns 3 and 6.