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.
Start with 2-3 co-founders and use other people to help get things going. They might do it for fun, a little cash, a little stock, or just the promise of bringing them in when it gets big.
Ask yourself if all these key people are really founders, or just the first employees with stock options?
This. Pick 1-2 "key people" out of the pool, and invite others as employees. Don't make things ambiguous, you should prepare and have everyone sign simple contracts outlining the conditions.
I am the tech guy, but I am.also the most experienced in the field, so I have some power too. But overall the structure is: CEO leads, I help. The others keep us two checked.
Never we made we tried to.make a full blown decision by everyone.
Only one of the 5 is really clearly a founder (the one with 50+%). The other two are somewhere between "co-founders" and "founding employees". Especially since you acknowledge that there is an inequitable division of power/decision-making. In a situation with co-founders in the purest sense, all co-founders would have equal power. And then you have two investors, who are not co-founders.
The two "co-founders" with less than 50% are "co-founders" in the same sense that Dustin Moskovitz, Andrew McCollum, Chris Hughes, and Eduardo Saverin were "co-founders" of Facebook. They were co-founders officially, but they were obviously 2nd to Zuckerberg and had far less power than he did (and none of them are at Facebook anymore) and were probably more like founding employees.
At most you could say you have 3 co-founders. But really the closer truth is that you have one founder.