Yes, it will mesh very well with what you are currently offering, plus protects your clients and yourself. I believe that it will work out great for you.
Here is where I totally agree with you. "Only offered to previous or new clients." My problem with multiple divisions is any time spent on one side of the business could have been spent building the other side. You lose focus. Your new division is a product of your hardscape side, so you still focus all of your energy on becoming better & growing the hardscape.