can all flashes be used on all hotshoes?
short answer: yes - at least in the really basic way: set flash sync speed on the camera body, and it will trigger the flash when you press the shutter, you sort-of adjust the exposure by setting the power of the flash in relation to available light and the f-stop you're using and your film's ISO. In real life it's very much an Arthur Fellig approach.
When you want the more fancy stuff things are getting much more complicated (and expensive) quickly.
I want a tiny flash
Compacts are great but the flash photos are never amazing like they are with a flash on an SLR.
I think if you use a tiny one (read: low-ish power, shooting straight ahead, no tilting / bouncing etc.) then the results do look kinda close to what a compact gives you actually.
For me at least the big change was when I got a proper powerful (read: big) flash, and also with bouncing the light.