According to HP, a full Unified Functional Testing license (UFT license) that you have which allows you to create GUI or API tests will allow you to use LeanFT on the same machine (if you are using a seat license).
A concurrent UFT license(s) allow you to use either UFT OR LeanFT in mixed usage (as in have some UFT full clients open and LeanFT in use) at the same time up to your allotment of concurrent licenses.

HP does also sell separate LeanFT only licenses if you wanted to purchase additional cheaper licenses just to use LeanFT if you wish.