Analog film photography and cameras

I would like to reduce it to allow more ambient light. Can one only "cheat" by adjusting ISO and +/-2 exposure compensation?

You’re going to want to read manuals specific to the lights and cameras your using. Exposure comp dial will usually manage overall exposure in TTL flash modes I think. If your flash / camera doesn’t do TTL then it’s keep it simple, forget that dial. There are a couple of basic things to know I guess:

Your sync speed is the fastest shutterspeed the flash can sync with. You can use slower. To use the flash as fill light (letting ambient light in) you can use a lower shutter speed with a lower flash power setting. You might be surprised at how little ambient light is needed before even a powerful fill flash disappears so the result is subtle but really nice. Try the guns lowest manual setting and exposing almost as normal.

Google guide numbers (GN) for flash power if you’re not familiar with what they mean. Then you can use the flashes manual modes. It’s worth having in your head as it relates equally to iso. Exposure = (distance dived by power)

Not really sure about the old flash modes for the TLR cameras.