requesting Nimslo (third-party) flash protips

I got a random cheap speedlight from Amazon today (Neewer TT560 Speedlite) and I'd like advice on using it.

I've had my Nimslo for a while and I shoot film recreationally on a few other devices but I've never explored flash photography before (other than the built-in flash on one of my SLRs). I understand that when a flash is attached to the Nimslo, settings are locked to f5.6 and 1/30 and that I am to use my speedlight's manual mode.

~~How do I know what flash power setting to use? Is there a science to determining flash power settings or is it an art of guessing and testing? I intend to test it out tonight on various settings but of course I won't see my results for a while.~~

*edit: Since posting, I've learned about guide numbers. Using* [*this calculator*](http://dpanswers.com/content/genrc_flash_calc.php)*, I entered my guide number, ISO, distance, and chose an arbitrary flash power such that the computed aperture was close to 5.6, because our aperture is locked to f5.6. Is this how I should be doing it?*

What kind of film should I use? I usually use cheap 400 speed drugstore Fuji film. Should I keep using 400 ISO? Also, I think I read that some people will mess with their camera's film speed setting when doing flash photography e.g. setting the camera to 100 ISO when using 400 ISO film. Is there a good reason to do this? What exactly does the ISO switch even do?

less-important noob question: my speedlight's spec sheet cites a flash duration varies from 1/300s to 1/20000s. i assume these are normal values; does it really not matter that the flash duration is so much shorter than the exposure time?

and if you have any other protips on nimslo/flash photography i'd love to hear em