When speaking precisely or technically, one would say that "Homo erectus and homo sapiens are two species of hominid" rather than "Homo erectus and homo sapiens are two species of hominids." The hominid here should be singular because we are speaking about instances of a single class ("class" being used here in its broader sense, not in the sense of taxonomic grouping).
Now let's consider more common parallel constructions replacing the word species with other words.
Tide and Wisk are two brands of detergent.
Tide and Wisk are two brands of detergents.
Cats and dogs are two types of pet you can buy at Pets-R-Us.
Cats and dogs are two types of pets you can buy at Pets-R-Us.
I feel that using the singular noun to specify the class is more grammatically correct, but sounds stilted in conversation. Almost everyone I hear making these constructions uses the plural form. What are your thoughts about singular vs. plural here?
Here's a more extreme example that may help clarify the issue. Consider the following sentence:
It was my first attempt at calming an angry crowd, and I just stood there while people were hurling all kinds of insults at me.
Here I think the singular insult sounds strange, if not downright wrong.
I just noticed in A Treasury for Word Lovers (Morton S. Freeman, 1983) a section entitled "Types of Errors." This is a book about (American) English usage by an English professor and editor, which purports to be a "practical guide for serious writers and readers." I wish I could say this discovery satisfies the question, but in fact I now feel somewhat farther from the truth, if there is any single "truth" in this matter. But at least I don't have to worry about being wrong when using the plural form of the class in these constructions.