Its occurred to me that in order for to help better explain what Im looking for (in terms of cell shading in CSP), I should explain my shading process in excruciating detail. I hope I can post this all...
For cell shading in photoshop, I've been using the following tools
-Brush
-Bucket
-Lasso, Polygonal, Magic Wand tools (for saved selections)
-Pen Tool
Even though my process has been shaken up over the years, I've mostly used the pen tool to create the shading and shape outlines. I also use for isolating certian areas that a saved color selection would fill in with the wrong color; pretty much areas not isolated by inked lines. I mostly use it for characters like this one:

As you can see, the area around his eyes is white, but as you can also see, there arent any inked lines to separate that from the blue. So a selection made by the wand tool or going in with the bucket tool wouldn't work, especially if I want to keep that area isolated during shading. So I would go in with the pen tool, create a vector shape in the area I dont want to be blue, and when I start filling in those areas, I can right click on the saved path, click 'make selection' on the drop down menue, and then click "subtract from selection". I also use this type of method for shading, where I'll also click "Intersect with Selection". This is pretty much how I've done my color/shading for a while now. As for the bucket tool, I mostly used it as an optional way of filling in stuff while not using the keyboard commands for fill. This goes way back before I knew that by checking off "all layers", you could use it outside the line art layer. The brush is mostly for making small additions to the shading but I only started using it recently as a replacement for the pen tool for my latest episode which was in color. A lot precise but very tedious.
But this method wont work in CSP due to the lack of any pen tool (at least photoshop variant of it. And while you can save selections via selection layers, I dont know of a way of adding onto them without making a new selection layer. This issue is only relevant now since Im making a new cover for my series in it, but it isnt the only thing Im struggling to grapple with or get used to in CSP