Therefore, how can I take into account and visualise in my sunny 16 evaluations with different ISOs, for example if I have 400 ISO film speed?

A film's ISO rating indicates its sensitivity. ISO 400 means that the film is twice as sensitive as ISO 200, and four times as sensitive as ISO 100. The sunny 16 rule says that on a sunny day, shooting at f/16 and a shutter speed that's the inverse of the ISO will give about the right exposure. So, if you're using ISO 400 film, f/16 and 1/400s will give the right exposure. If you have ISO 1000 film, f/16 and 1/1000s gives the right exposure. The shutter speed is adjusted according to the ISO value because film sensitivity and exposure duration vary inversely: if your film is twice as sensitive, you should expose it for only half as long.

Sunny 16 doesn't mean you have to always shoot at f/16, though -- it's just a guide to get the right exposure value. Once you know the right shutter speed at f/16, you can easily determine the shutter speed at other apertures because aperture and shutter speed also vary inversely: if you use an aperture that's twice as large, you're letting in twice as much light, so you should expose for half as long to get the same exposure level. Conversely, if you decrease the size of the aperture (remember: that means the f-number goes up), then you have to increase the exposure time. For example, if you want to shoot at f/8 instead of f/16, you'd compensate for the larger aperture by reducing the exposure time by two stops because f/8 is two stops faster than f/16. Or, if you wanted to shoot at f/22, you'd increase the exposure time by one stop because f/22 is one stop slower than f/16.