You need to set up masks, ie black and white coloured ouputs into surface layer's blending input, be that from power fractals, images, or sets of rules you've created with the node network. Black equals nothing and anything up to white is apply what ever you want.
It can get complicated the more rules you put into a scene. It would be rather good if you could set up channels/layers within TG which would be one of my top 5 features. In the meantime, it's a good idea to name your nodes, eg "Field 1 Mask" etc so that you know what's doing what.