yes it is.
Cnidarians are the simplest organisms that have attained a tissue level of organization. They consist of little more than an outer covering (the epidermis) and an inner gastrodermis with a non-cellular mesoglea sandwiched between the two. Mucus-secreting cells cover the animal with a protective slime. A single opening serves as the entrance to the gastrovascular cavity, within which prey is digested. Lacking an anus, any undigested material is ejected through the mouth. With most of the living cells in direct contact with the water, specialized excretory, circulatory, and respiratory are not needed. Nonetheless, the nervous and sensory systems are reasonably well-developed and many cnidarians are capable of quite complex behavior. All members of this phylum possess stinging structures called nematocysts housed in specialized cells (cnidocytes). Cnidocytes are especially concentrated on the tentacles, but are also found scattered over the epidermis and gastrodermis. In some species the poison contained in the barbs of the nematocysts is powerful enough to harm humans (most notably, those of the Portuguese man-of-war, Physalia). Most, however, don't penetrate the skin or if they do they cause only slight irritation.
Now to say that you can't have more than one cnidarian is kind of wrong. Better to say that you should not have more than one species of anemone in one tank. Otherwise we would not put button polyps or zoos in the tank with other zoos or anemones, and most of us do that all the time. Many types of cnidarins can be in the tank, you just need to know that they can sting to some extent and they might not get along when in close proximity to each other.
Thomas