Are your cages built? I would think you'd benefit from having a larger space (6x6x4). The length of your juvenile muskies will likely be approaching 50% of the "run" you're planning fairly soon. Given Bill's advice on getting them to eventually strike what's dropped in their cage, you'll want space to avoid injury from other fish and the cage itself.
Not a perfect comparison, but I can't imagine feeding/growing five young 6" LMB in 20" cube.
I'd love to see you get it to work out so I may someday copy you!
