I upgraded to Nuendo 15. I have Spectralayers Pro 12 and Wavelab Pro 12.
Will it be necessary to install the “Go” versions of these last two plugins to use them in ARA; Do they have something different compared to the Pro versions that I have?
As I unterstand it, it´s the same install for all versions.
Depending on which one you own and have installed and licensed in the authorisation manager, it´s that version that will get initiated in Nuendo.
Like you I have Spectra Layers Pro and Wavelap 12 pro and everything is fine in Nuendo.
The pro versions are the most featured, so no need to install Go.
No, you don’t have to install them. These Go versions offer very basic features that are already included in the full versions. ARA extensions will automatically pick the highest version available.
EDIT: @wolfger beat me to it, a minute too late 
Thank you for your response. You are very kind.