No point in ruining a good chisel
I'd just use another hard rock and gently tap it against the sharper corners of those rocks to make it less pronounced and to have the outcome be a bit more random. Should be able to gauge fairly quickly how hard of a 'tap' you'll need to give it. So long as the edges aren't so uniform, I think the flatness of the side will be less noticeable. Can always try on one of the smaller pieces and see how it goes.
If the rocks are slate (and if not they look similar) then hitting them with anything probably won't rough them up. It will more likely split them into more flat pieces with edges the same as they have now. That is what has happened to get them of the size they are now. Unfortunately that is nature if rocks like this.
However, I don't think they look unnatural at all. I have rocks that look like that naturally where I live. I think they have enough character to use them as they are. Sure they might look better if they were rougher, but I doubt you will be able to do it.
As far as the layout goes. I like parts of it. Actually I like the whole thing but I'd try moving it. I would swap the two groups of rocks though. Exactly how they are just the right group on the left and the left group on the right. I think that would add a lot more drama. If you can though try and keep the 'gap' between them off center.
It's personal preference and you will find tanks with the rock pointing both 'in' and 'out' but I prefer the rocks pointing in kind of like the tanks below.