Well they don't have to mix, though it's an interesting crossover.
Not all humans need to die in Interfector's opinion, they just have to surrender and submit. Humans aren't very good at that lol.
Often a cheaper option than getting a full commission is buying a base. It's like a coloring picture.
Basically you buy the rights to use the lines from an artist and you can color them yourself, this is an example:
This way the most difficult part is already done. They will probably send you the photoshop file and you can neatly color and edit it using the layer system.