Okay, without a practical example I can't show you how to do it. So I'll try to explain how it works.
Think of it like a kids' coloring picture. Slice Transforms are the black and white outlines of the shapes that make up your picture. They prevent you from coloring outside of the lines. But you still have to decide how to fill them.
So you're going to need to do some scaling yourself. Every stage is different, every look is different. So you need to eyeball how to scale your content on the stage. Depending on your content, a stage with very wide wings needs a different scaling than a stage with lots of thin vertical pillars. There is no way to automate this. In fact, deciding which looks work on a particular stage is what sets a good VJ apart from a technician.
If you really, really want to automate this, you can use the Auto Size mode on the layer, set to Fill. Combined with Slice Transforms set to Fill, this will make sure your content will always fill the slices you're using it on. If your content is mostly just abstract moving wallpaper, you can get away with this. If you're trying to tell any type of story, you need to make sure your story is visible. I would always use my programming slot to go through my content and my looks, and see if they translate/transform well to the particular stage I'm playing that night.
Practically, I would use the single default transform on the clip to adjust each clip to taste. Using Slice Transforms on the layer, I would then decide which parts of my stage that bit of content ends up. I'd throw out the second transform you're applying everywhere. It doesn't do anything the first one couldn't also do, and just adds confusion.