Unfortunately, I think you might be stuck using one or the other. From the looks of it, rokSlideshow is dropping the mootools js it needs to process right into the body of the page rather than posting it up in the header (where you could get rid of it). See if there are settings inside the admin part of the rokSlideshow module that allow you to turn off the inserting of the javascript. Then see if it all plays nicely together.
If you CAN turn off the javascript insertion and they still don't work together, you might want to try building your own mootools.js file over at the mootools.net site... but that's a little extreme if you're just fumbling around. Although if you remember to rename your old js files instead of overwriting them as you try different combinations, it's easy to back out. That's how I got everything working on my site.
I know it's not much of a help to say, but you're not the only one that runs into this problem. These forums are littered with people trying to get various flavors of the rokmodules to work together. Probably the easiest answer would be to pony up the $20 bones and get monoslideshow and use that. Slimbox and monoslideshow play nicely in the same sandbox.