I’m also downloading Storyboard 6.
If I keep Harmony 17 Premium, will I need Storyboard 6?
I’m trying to make a 30-minute animated cartoon. Thanks. 
There’s no reason not to storyboard in harmony itself.
To be clear and not potentially mislead anyone there are no storyboarding-specific features within Harmony unlike some other animation software such as TV Paint. You could use just about any graphics software to storyboard. Some would be better suited than others and there are reasons to use something other than Harmony itself.
Based on the OP’s words it sounds like a one-off dream project. If this is the case time and energy learning two programs is unnecessary. If you are taking steps to create that dream cartoon you probably have experience drawing on paper. It would be a quick and familiar process and there is a lot of flexibility with index cards. A simple approach using the tools within Harmony would take additional time and effort to adapt but that depends on the person. If you are new to the software you are better off going at it with a singular objective. Later you can expand your knowledge and skills. Of course ultimately the choice is yours.
Harmony does not have built in storyboarding ability.
You do not “need” Storyboard 6 but it would be useful if you want to storyboard your cartoon.
You could storyboard using index cards. You could even storyboard using several sheets of simple paper with multiple rectangles drawn out on each sheet. It really depends on the amount of flexibility you desire .