For the best tasting ceviche dish it's best to use fresh fish or fresh sushi if possible.
However if you cannot get any fresh sushi then previously frozen fish is perfectly fine to make ceviche with and no harm will be done other than the taste may not be as good.
It's very safe and perfectly fine to use shrimp, and scallops as well as any fish that has been previously frozen to make the ceviche with.
Just defrost the previously frozen fish first and then toss it in a citrusy marinade, and then place the mixture in a refrigerator for a few hours to let it setup and be ready to eat.
Sometimes Ceviche is ready in as little as 1 hour but if you let it sit a bit longer it will taste better.