You are in fact correct. The reason you can't have scatter and categorical series on the same chart is that the series rely on the axes to be rendered on the correct position.
This means that when using categorical series you need one of the two axes to be CategoricalAxis and the other - linear or logarithmic axis. And as you probably know Scatter* series need both of the axes to be numeric.
As you have already figured out yourself, if you need a point series to be displayed in a categorical chart, set the line color to transparent and use a point template.
the Telerik team