I've used square tube for the outside tube on sliders...it worked fine. Gave the slider a sort of "geometric" look that some might dislike. Plus, one could say that the 45º angles might be more prone to catching on stuff. I'm not so sure. There are lots of 45º and 90º angles on all kinds of stuff underneath the Cruiser, so I guess it comes down to aesthetics and preference. I would, however, strongly suggest going with a thicker tube. On the set I made, we used 3"x2"x5/16" rectangular tube on the slider's main stay, then 2"x2"x5/16" square tube on the outriggers and step. If you're going to lay the weight of the truck onto a corner of tube instead of the side of a round tube, make sure it's burly enough to cope.
One thing you might consider if you're doing your own sliders is doing away with the secondary tube altogether. Just extend the outriggers past the body pinch and have the slider do dual function as a step. We did that on a set for my Xterra and it worked great. They flexed a touch, but that actually turned out to be a good thing. Had them for a year and a half, abused the hell out of them, and they stood up well. I sold them when I got rid of the Xterra. Some guy in Nor Cal is still 'wheeling the crap out of them. Here's a pic: