Can I substitute table salt or sea salt for kosher salt? 1:1?
as long as the 1:1 substitution is by weight. kosher salt is lighter per volume than table or sea salt. A general rule by volume is this: if you are using Diamond kosher salt, you will need half as much table salt. if you are using Morton's kosher salt, you will need 3/4 as much table salt.
Sea salt would be preferred as it's coarse salt whereas table salt is fine and also contains things like "flowing agents". I wouldn't worry about ratio so much because you are going to taste for salt, right? Also the stuff under the "M" lable is mined while sea salt is harvested. Although it's all sodium chloride sea salt also contains traces of minerals that enhance the flavor. But it depends on the dish you are preparing which so far is unspecific.
I always use extra Kosher salt than what a recipe calls for when it just lists salt, however, I did not know that there is an exact "science" to it. Do Morton's and Diamond's crystals vary a great deal, enough to cause the different ratios? Or is it a taste difference?
Yes. Morton's weighs about 7.5 oz per cup, Diamond weighs about 5 oz per cup, and table salt weighs about 10 oz per cup. If a recipe just calls for salt, that typically means table salt.