Adding electrical service to detached garage
I'm not entirely sure what you mean by "neutral and ground to be isolated" but it's my understanding that they should always be connected so that there is zero potential between them.

My understanding is also that only connection between neutral and ground should be at the main panel.  If you connect the neutral to the ground in a second location (the garage) then you'll create a "ground loop".

I'm not an electrician and it's been about 10 years since I read the electrical code so I could be mis-remembering.