That is an excellent question, from my personal experience its better to master 1 subject, weather a creature, human, or an animal. once you understand 1 animal and how there bones, structure, and anatomy works its a lot easer to pick up others as you will pick up on all the visual clues, anatomical similarities and drastic differences to your base. With practice you'll get more comfortable with your subject and pushing there expressions will be a lot easer as your less focused on getting it to look structurally right. Granted for me that was only once I had a pretty good grasp on my own subject that I could really play around with pushing expressions, believability, and dynamics. I've personally approached creature design as a work your way up sorta process as your skills and imagination grows, start with just combining 2 subjects in a believable way before pushing your self to tackling more complex combinations requiring you to solve challenging conundrums like how to blend fur, skin, and scales together. all in all really love your concept, and this is just what I can say from my experience and what worked for me, it might not work for everyone's learning style so consider it with a grain of salt, hope this helps answer your question.
