The relationship Torque=(Moment of Inertia)*(Angular Acceleration) involves the term "angular acceleration". Obviously we can talk about angular acceleration of a system of particles about an axis only if all the particles in the system have same angular acceleration. Otherwise the term "angular acceleration" itself won't be defined. Thus, in general, angular acceleration is defined only for a particle or a rigid body (and not for a system of loose particles, as the angular acceleration of different particles will be different).
We also know that the given relationship is true for any rigid body, with respect to an axis passing through the body. The given relationship is trivially true for a particle.
Hence, the given relationship is true for a single particle as well as a rigid body.