a few MTS never hurt anything! Great to sift the gravel and keep things healthy under there. And if their population takes off, you know you're overfeeding the tanks, so another good way they can help keep things healthy.
I second the idea of making a river rock river rather than trying to do it with sand or a different colored gravel. It's REALLY hard to keep things separate even from the start and nearly impossible in the long run. I think the rocks will do just as well and create the look you're going for.
I'm always cautious about adding plastic to tanks, not really because it causes harm, just makes things look very unnatural if you can see any of it at all. You could perhaps get a similar result with thin pieces of slate rock? But that could just be my bias