In short yes your original (grey) cab-1 and base will operate a LEGACY locomotive.
LEGACY is basically the second iteration of TMCC it was actually originally marketed as TMCC II
while you can run a LEGACY locomotive you do loose a lot of the LEGACY specific features such as quill-able whistle
also just as a FYI the Cab-1L (blue in color) works just like an original cab-1 slightly more refined has a poor man’s ability for quillable whistle and only talks to the LEGACY base-2 or the new BASE-3