In my opinion I agree with SlideRule... When I have to have foundations close, I still generally don't tie them, But if bearing on granular soil I will inject the bearing soils below both the edge of the new and existing (if mat) to a common depth/elevation with chemical polyurethane grout/soil...
For me I would give the contractor a choice, they can go shallow but I would reinforce the slab to make it stiff enough to withstand the seasonal differential movement and require all fittings/connections to the generator be flexible (no hard pipe). Or put it at a depth you are comfortable that...
