I have been researching the controller marketplace ( like you I see on this forum 

 and dont own the Nucleus (but am interested in it too).
The interface is completely separate from the controller aspect and in fact you dont even have to install the sound card drivers if not needed according to the manual.
You simply have to add it as control surface to your DAW setup. Your Apollo of course is not a control surface - so no conflict.
I have also seen on other blog posts that you can use the SPDIF output from the Nucleus and go to the ADAT input on your your Apollo with a coax/Toslink converter.
Doesnt sound like you are interested in the sound card but for some time its been a bit of pie in the face of SSL that sound card does not work on newer macs with haswell chipset and El Capitan. They have a note about it on their site.
Quite an inspiring looking console I have to say.
Nice to have some owners of Apollo's and Nucleus to pipe in on their experience.