Composing custom tables in Mathematica is still something of a problem because the principle construct is Grid and this depends heavily on a somewhat convoluted Option structure. Although this is not a direct answer to your question, which I don't completely understand, the Presentations Application, which I sell through my web site, has a TableMaker section that allows one to construct custom tables in the same manner that one might draw a graphic. It interactively translates table composition commands to the Grid structure. It is also possible to extract the final Grid code to use in notebooks not using Presentations.

I have attached a PDF file illustrating an example.

Attachments:

The Table style is defined in the stylesheet named PublicationDefault.nb, which is referenced by the various article and book stylesheets. The key option in effect here is this setting for column item size:

GridBoxItemSize->{"Columns" -> {{
      Scaled[0.503]}}, "ColumnsIndexed" -> {}, "Rows" -> {{1.}}, "RowsIndexed" -> {}}

making each column 50% of the window width. This is really only appropriate for 2 column tables, and should probably be reconfigured to something a little more practical. To override the setting, you can apply the following at the cell level:

GridBoxOptions->{GridBoxItemSize->{"Columns" -> {}}}