Is there room to build another wall with a space between it and the house wall? A large enough space to get a rake in to clean out debris, at least one foot wide. Other than not wanting the animal run right up against the house, I don't see any downside to having a concrete floor, as long as you put a roof over to protect from rain. This way you will be able to control the moisture level of the compost. As far as soil life, simply adding garden soil periodically will take care of that. Eventually the compost will develop its own ecosystem. I envy you having so much of a composting animal system already built!