Ok, as for Fill Layers:
Why use Fill Layers at all?! That's for kids...
There is much better solution for such things: add any adjustment layer, say Levels with default settings. Now go to Layer Style dialog for this layer and here you will find everything you need:
Color Overlay style to have Solid Color fill
Gradient Overlay style to have Gradient fill
Pattern Overlay style to have Pattern fill
All in one place and easily manipulated. I usually use them in combination: add Pattern Overlay with some base texture, add Gradient Overlay in Soft Light mode to add some realism, use Color Overlay in Color mode to add the color touch I want for this particular case.
And there is much, much more here: shadows, texture, emboss, stroke, satin...(one minor point though: if PS created your Levels layer with the mask - discard this mask and make your own one instead - Select All, Add Layer Mask. Don't ask why - I don't know, but with the PS supplied mask some styles(like stroke, etc) just don't show up)
Now back to your question. All the same - place the layers in the group, change the group blend mode to normal. Now add the Levels layer to the group the way I described earlier, use all the proper styles to get the effect you want.
And one important final touch: check the "Blend Inferior Effects as Group" option in the Blending option section.
Here you go: forget the Fill Layers, use adjustment layers with styles instead, set "Blend Inferior Effects as Group" option
