That's gorgeous Watson!
You don't have to use tulle unless you want to. If you do, then I suggest you use a black tulle as the darker the tulle the more it fades into the background and it is less likely to change the apparent colors.
You could also use a very fine (like 100 wt.) thread to stitch around all the pieces to hold them in place. I use Invisifil thread (100 wt. polyester) it's very nice thread and my Sweet Sixteen handles it very well. It comes in a variety of colors, but I think you could get away with just a black, a silver/gray and a red. I think I recall that you said you'd purchased a Tiara? if so, that's pretty much the same machine as my Sweet Sixteen. If you have the horizontal thread spool attachment, then you will need to use that if you get spools of invisifil--- the thread on the spools snags on the ends of the spool if you try to use the vertical thread holders. If you don't have the horizontal spool attachment then get the cones (they come in a mini cone) and those work fine on the machine's vertical thread holders.
Rob