Riccia is a great floater. If you give it some lights and ferts it will explode in growth.
Plenty of live stem plants are easy to grow. My milfoil is like a weed, and rotala sp. green and ambulia are both quick spreading as well. I prefer live plants as they sort of 'grip' your females when they swim through, but don't scratch them like a plastic one might.
Also java moss is absolutely great for sororities. I had a big ball of it, and one of my quieter females would rest in there when the others got too rough. If they wanted to reach her, they would really have to work at it.
Forgot to add, I look forward to seeing photos of your sorority. They are very entertaining to watch, and if done right from day one, not hugely stressful at all.