If 'carnivore' term can be used for cnidarians then can 'omnivore' be used for sponges ?

Yes, sponges are omnivorous. They feed on bacteria. See info here (http://true-wildlife.blogspot.com/2011/03/sponge.html).

"Omnivorous" is the feeding strategy where an organism feeds on other organisms from at least two of the different kingdoms of life, like the bacteria, protists, algae and especially plants and animals. An organism that eats plants and animals can be called omnivorous. The website (http://true-wildlife.blogspot.com/2011/03/sponge.html) states that sponges eat phytoplankton, bacteria and bits. Since bits, phytoplankton and bacteria are from different kingdoms, the omnivorous description is appropriate. "Carnivorous" means that an organism feeds only on animals. The website talks of species of sponges with a more carnivorous diet, as they feed on fish and crustaceans. (The website not clear whether the sponges can also eat bits.)

Cnidarians are the jellyfish and corals, and most of them are carnivorous, but there are corals, who are completely dependent on a symbiotic relationship with dinoflaggellates to assist them with photosynthesis of their energy, making them essentially just like plants.

Organisms that feed on plants are called herbivores. Plants can also be carnivorous!

Your if/then question is a non sequitur.