At a tappas bar, I had shrimp ceviche. In this dish, the shrimp is "cooked" not by high temperature but by the citric acid of its marinade. This would be really fun to make at home. For such dishes, should you stick to tested recipes? Or is there a way to figure out: what kind of acids will work, what kinds of meats will work, and when the meat is sufficiently "cooked"?
Edit: Added quotes around "cooked" since, as Bob pointed out, the meat is not cooked per se.
Ceviche is not exactly "cooked", but the acid causes the proteins to become denatured in a similar way. It may not kill all bacteria and parasites as effectively as cooking (with heat), so like sdg said, it's safest to use food that you would eat raw.
"Sushi grade" can refer to the fat content of the fish (like the USDA grades for beef marbling) rather than an assurance of its safety, so don't rely on that to pick out ingredients. Saltwater fish should be frozen at sea both for freshness and to kill parasites, and I prefer to get individually quick frozen (IQF) shrimp both for their freshness and convenience. Saltwater fish at your local supermarket has been previously frozen, so you're usually better off just buying frozen fish and thawing it yourself, unless you're really pressed for time.
Freshwater fish should not be used in a raw preparation, including ceviche.
Whether or not you should stick with a tested recipe depends on your expertise and the food in question.
In this case, if you are an experienced chef, and the food you are preparing you would be willing to eat raw (i.e. sushi grade), then no worries, just go ahead and experiment.
On the other hand, I tend to stick to a recipe pretty closely for the first few times. Once I understand how the dish should turn out if I "follow the rules", that then gives me guidance as to how I can break them.
Good Luck
