Is Fancy Feast good?
Judging by the nutritional information of both 'liver and chicken feast in gravy' and 'purely Fancy Feast', this wet food doesn't provide enough fat. Fat is the ingredient that 'tells' a cat's brain to stop eating at some point and of course it is also their main energy source. This means that they need a lot of this type of food to actually be full. Additionally there seems to be a lot of liver in the food (at least it's in the second position on the ingredient list). So no, not great.
Dry food should not be fed as a full meal either because it's dry (cats naturally aren't great at drinking a lot) as well as usually filled with wheat or other stuff cats can't even digest properly.
Can I feed Tuna?
Sure. However, fish is a special thing even among RAW diets. Fish has a ton of Vitamin D which can be 'overdosed', so to speak. Cats won't die from a fish every few months or anything, but it's not great for them either. Additionally, some fish are rich in Thiaminase (https://en.wikipedia.org/wiki/Thiaminase) which breaks down thiamine (Vit B1), which in turn means that it will effectively remove that from whatever else the cat is eating at the time.
There have been studies where cats being fed fish already showed deficiencies after 11 days (Thiamine Deficiency and Associated Clinical Disorders by McCandless, 2009) symptoms of which can be loss of appetite, nerve damage (trembling, cramps, ataxia) and overall weight loss.
This all might sound very scary indeed, but again, non-supplemented fish or other meat once a month or something is perfectly within the range of 'fine' :)