It is cool. But is it useful? The only information it gives is day of week, assuming you already know where to position your slider. It doesn't have holidays and number of days in a month. So if you don't already have a calendar with all the information, you can't use this calendar.
A show is like a dream. There are so many images, so many ideas that cross my mind during a shower. But they can be just as easily forgotten as a dream if you don't deliberately memorize them after the shower.
I have no experience on hardware startups. But my gut feeling is that they do have additional layers of complexity relative to pure web startups. As such it makes sense to have somewhat more co-founders than web startups.
However, the more people you add to a team, the slower it moves and the team dynamics grows exponentially. It will reach a point after which you get diminishing marginal gain, or even negative gain.
For a startup I believe agility is critically important. I would be worried to add too many people from the outset because it is difficult reverse it. It's hard to 'unfounder' a cofounder later on. My advice is to have a core team of 2-3 cofounders to see how it goes. If you really feel insufficient after a while, bring one in one at a time. It is less risky than having too many in the beginning and discover problems when it's too late.