Honestly. For something such as this I wouldn't bother with dispersed boiler and machinery spaces. The Arethusa was IMO the smallest workable 6" cruiser. The way you're going you're going to end up with more of a super destroyer than light cruiser!
1) would a super destroyer not want dispersed BR/ER as well ?
2) why cant my 6 inch CL work ? (say at 4500t ?) (my only thought is that 4 guns doesn't give good salvos)
3) maybe would 8-10 4.5s (or 4.7s, I don't like the idea of loading 5.25s on a DD) on a super destroyer (3000t) be better ?
JSB