I agree. There's no magic number. Small fish, and fish that evolved in lakes or the backwaters of slow moving rivers aren't likely to appreciate fast moving water, while larger fish, especially ones from fast moving rivers just might.
Substrate and food matters as well. Gravel will stay put under fast moving water, while light sand probably won't; flakes could disintegrate, while whole shrimp won't.
Of course, some of this can be managed in how you add the flow to the water (surface or subsurface, etc.), but I think you get the gist.
If you can identify the fish you are getting, you might get more useful information.