It is, yes. If you had a tank with two different catfish species in, 'catfishes' would be a valid term to use, for example. "These catfishes live together in the tank."
The word crises is a plural word; it is the plural form of the word crisis.
They are harmless.
There is no plural word for if.
The plural form for the demonstrative pronoun this is these.
A non-plural word, a word (noun or pronoun) that is not plural is singular, a word for just one.
The plural word of delay is delays.
the plural word is comedones
No it's a singular word. A plural word would be "have".
Leukocytes is the plural of leukocyte
The plural of "classification" is "classifications."
The plural noun is raisins.
The correct plural for the word "wolf" is "wolves."