Fish oil is typically fish body oil from unspecified fish. Salmon oil is, indeed, salmon body oil. Cod liver oil is has a completely different nutrient profile and is high in some oil based vitamins that can accumulate in the system.
Stick with a good, reputable salmon oil, or even anchovy and/or sardine oil - the best sources for EPA and DHA.
How much you give depends upon how high the EPA and DHA contents are in the particular oil you are giving.
You should also supplement with vitamin E if you are giving fish oil. Doses of each do depend upon the dog's body weight.
Depending on what you are trying to achieve with giving the oil, as I said above, stick with a good, reputable fish body oil. Check around, too, because some do already have vitamin E added to them.