Is it typical that smoke detectors in adjacent apartments be linked?
It depends on your building/fire code jurisdiction, some do and some do not.  More importantly, if the garage is a separately occupied dwelling it should be isolated from the other apartment via fire-resistant materials.  This presumption (which is almost always a part of the local building code) is what alleviates the need for an interconnected alarm in the practical sense.  If it's not, I would sleep well knowing that a fire in the other unit would trigger the alarm and wake me up.