You usually get smoother results if you apply Plugin Galaxy to a large sized photo before you scale it down to the final output size. Same applies if you try to generate textures from scratch with Plugin Galaxy. If you want the texture to be 500 x 500 pixel large in the end, you should better start with a 1000 x 1000 (or even 2000 x 2000) pixel image and size it down after the texture was created.
You can selectively apply a Plugin Galaxy effect if you create a selection in your graphics application before running Plugin Galaxy. Then only the selected image area will appear in the preview and only this image part will be changed after pressing OK. Often it is a good idea to feather the selection to make the transition between the effect and the rest of the image as smooth as possible.
The Blend combo box and slider below the effect controls is a nice way to blend the effect with the image, especially if you want to use Plugin Galaxy effects, which create texture effects, with normal photos. You can also try different blend modes, e.g. NegDif, Darken or Overlay, to achieve special effects.
Plugin Galaxy 2 lets you apply more than one effect to the image without leaving the plugin dialog. To do that you just need to switch to the Layers list in Expert or Manual Mode and press the New button to create a new layer with a new effect. The new effect will be applied on top of the previous effect(s).
On the Presets tab sheet in Expert Mode you can find the Keep Layers check box. If this check box is deactivated and you open a preset by clicking one of the items in the preset list, then the old effect will be replaced by the effect of the preset. However, if you activate the Keep Layers check box and open a new preset, then the new effect from the preset will applied on top of the old effect. This way you can combine the effect of available presets.