1. It's correct, you can use は but it depends on the context. 2. Yes, we understand you. 3. I'll explain about it with examples. When you talk about the food, for example about すし in this case, it's natural using が for the first time. After talking about it, comparing it with other food or if someone asks you about it, ect...there are cases that using は sounds natural. Examples are: When someone asks you if you like sushi or not, (お)すしは好きですが、たこは食べられません。 (I like sushi but I can't eat octopus.) After someone says that he doesn't like the Japanese food, 私はおすしは好きですが、天ぷらはきらいです。 (I like sushi but I don't like tenpura.) If someone invites you to eat sushi, 私はおすしは好きですが、そのレストランには行きたくありません。 (I like sushi but I wouldn't like to go to that restaurant.) I hope this was helpful.