This was posted in a forum thread about the anthology. I thought the information might be useful:
... and the answer was:
We've gotten a number of inquiries about the nature of the aliens attacking Beta City. Big? Small? Humanoid? Monstrous? Basically, the only thing set in stone is that there's more than one kind of alien invading at Kharsear's command. Here's part of an answer I posted on the Beta City thread at our publisher's website:
There are a few visual examples from the comics that might be helpful as well. Consider Kang's shock troops from the Kang Dynasty storyline. They're an irregular wave of armored, high-tech troops. Even better, consider them alongside their Deviant and Atlantean allies to get an idea of the kind of variety I'd like to see.
The armies of Annihilus in Marvel's Annihilation event are another example. While, yes, the army is mostly bugs, and many of them look the same, take a look at the different miniseries. You see groups of similar bugs together, but the different creative teams each have their own take on the troops. When the Centurions attack, you see the other end of the spectrum -- each individual is from a different world.
Finally, consider the armies of Darkseid from Kirby's Fourth World. Groups of parademons do tend to look and dress the same. But there's often some variety amongst groups from Apokolips, often with some giant contraption or creature thrown in for good measure.
These are just examples for illustrative purposes, of course. Please don't use another creator's property in your own stories. Feel free to contact the editor to discuss how you want to use the aliens, what capabilities you want to give them, or anything else!