On the Ninja400: I have never actually seen one... Not sure they were/are imported here. But I'm sure Kawasaki included a sufficient safety margin & with your weight you'll be fine. That's actually quite an ideal weight for a racer, 65kg. Wish I was a bit lighter, but over the years I gained...
On water & waterless coolant, there are 2 effects that play a role: boiling and expansion.
As we all know, clean water boils at sea level at 100°C (one of the gauge points of the Celsius scale, the other being clean water freezing @ 0°C). Dissolving something into the water (like anti freeze) increases the boiling point, and reduces the freezing point. When water (or any fluid) boils, it evaporates & turns gaseous, thereby taking up a much larger volume. Consequently, in a closed system, the pressure rises, which actually increases the temperature of boiling. Engines may run a bit above the atmospheric boiling temperature of the coolant, and the lot is kept in order by the higher pressure. Caps come with different blow-off pressures.
This is not what the expansion tank is for. The volume increase liquid --> gas is (depending on what it is) much greater, in the order of 100 to 1000. So if you have some 2L of coolant, you would need a tank of some 1000L to capture that in the gaseous phase, which is awkward.
A waterless coolant can be a mixture of glycols, and it still boils. But at a different temperature, say 190°C. If the coolant would reach a temperature like that, your piston has long since melted, so yes, you can operate at a higher temperature, but you'll also loose the protective effects of a coolant with more water (boiling at, say, 120°C @atmospheric P). It's a choice. Also/more important than the (atmospheric) boiling point is the heat capacity of the coolant. That determines how much heat you can transport from the engine to the radiators. Finally, note that glycols expand too, see
https://www.engineeringtoolbox.com/cubi ... _1262.html. Typically twice as much as water, actually.
A higher boiling point is for instance desirable if you're doing a difficult climb, making no or little speed while using the clutch a lot. That can get the engine boiling (and on my YZ I routed the blow-off tube up, so that I see that happening). But running the engine above the normal design temperature of course also comes with risks.
Expansion is something else. Most (if not all) materials expand when their temperature increases, both solid and liquid (& gas). If the coolant expands more than the metal (barrel, head), you need some room in the cooing system to allow that. It's not a lot of volume, and often a bit of air is sufficient. Or the manufacturer preferred to include an expansion vessel for it, wanting to prevent air in the radiators, for instance. Very common in cars too.