How could I say the equivalent of "A pouring wave of humanity," when there are more than one species in the scene being described?
How important is the sentence is your narrative? If not very important, then "everyone poured into the streets." Bad on you if you haven't set up the reader to understand that "everyone" refers to those of all species. If it is important, and this is perhaps the first time you are showing eqivalence amongst the species, then maybe something longer, like:
It was as if a fire has been set in the center of the city, and everyone began to flee. Those in the center fled first, and being mostly populated by XXXXXXians, they came quietly by the thousands, their cries unheard by the humans whose hearing did not reach to that part of the spectrum.
But, the fire grew and first tickled, then engulfed the other neighborhoods. Humans of course, but also those with denser skins, and even the FIREites who struggled constantly with the cold. Standing on the outskirts, where the density of buildings grew sparse and the fire was finally controlled, Biggees, Littlees, XXXXXXian, humans, and FIREites milled around, finding their friends, calling for family, and trying not to think about what was lost, or how life was so quickly changed.
Few words if not important, many words if important. Don't use too few words that are overloaded with connotations. Don't say the obvious. Use the words as an opportunity to push the story forward.
Use a general word that includes them all. "A wave of creatures", "a wave of beings", etc. If it's necessary to identify the creatures, be more specific. "A wave of creatures from almost every species of land animal on Earth ..." Or "A wave of beings from every planet in the Orion arm of the galaxy ..." or whatever.
You'll have to answer some questions.
What species are there?
What do they look like?
If the species look similar, you can use similar words. It also depends on the species' name. For example, "humanity" comes from "human." If you named a species "Roax" (I just made that up, it's not from a certain place), than you could call it "Roanity," or "Roanix" or something. Try to make them unique.
One you answer these questions and give them each their own form of "humanity," you'll have it. Also, in the sentence "A pouring wave of humanity," you could say something like "A pouring wave of humanity, roanix, and [whatever else]."