You should really mention that in the description that you're doing a copy, otherwise you're misleading and it's would also be a design theft... The design world isn't as forgiving about that as the anime community and people drawing from "bases", especially if you're working/intending to work in the field. Study of styles and wise designs is somewhat ok, but don't publish that stuff, for your own good.
I'm not surprised, they're amazing try to look through a lot of references (photos, landscapes,etc.) all within a short time - Your mind will most likely synthesize it more so Your art will be more original