I think Gatekeeper has a good point about the stems overshadowing your rockscape. That is what I experienced in my 40B as well. I've since taken out the big rock I had near the back because of this. You couldn't even see the rock after the plants grew in. You're rocks are smaller compared to the rock I had so they'd be swallowed in a matter of weeks with Rotala. It's def doable if you keep trimming the stems back. You're idea for the mid - foreground sounds good.