It really depends what else you use the lighter rod weights for. If you want a rod that will be good for kahawai as well then go with a six. You could catch flounder and yelloweye mullet on a zero weight but I find anything lighter than a five weight becomes a little bit impractical in the salt.

Addiction could turn to madness if you get fixated on grey mullet ...

Grey mullet are great for casting practice. The few I have caught have always been in those very active groups of fish that are milling about on the surface and almost look like they're chasing each other around. This gives you a very active and fast moving target to cast too. Plenty of direction changes and delicate presentations required. A stripping basket is almost essential. It's great fun. Although it can get frustrating!