In this contribution, we introduce a versatile formalism to derive unified two-phase models describing both the separated and disperse regimes. It relies on the stationary action principle and interface geometric variables. The main ideas are introduced on a simplified case where all the scales and phases have the same velocity and that does not take into account largescale capillary forces. The derivation tools yield a proper mathematical framework through hyperbolicity and signed entropy evolution. The formalism encompasses a hierarchy of smallscale reduced-order models based on a statistical description at a mesoscopic kinetic level and is naturally able to include the description of a disperse phase with polydispersity in size. This hierarchy includes both a cloud of spherical droplets and non-spherical droplets experiencing a dynamical behaviour through incompressible oscillations. The associated small-scale variables are moments of a number density function resulting from the Geometric Method Of Moments (GeoMOM). This method selects moments as small-scale geometric variables compatible with the structure and dynamics of the interface; they are defined independently of the flow topology and, therefore, this model pursues the goal of unifying the modelling of a fully-coupled two-scale flow. It is particularly showed that the resulting dynamics provides closures for the interface area density equation obtained from the averaging approach. The extension to mass transfer from one scale to the other including capillary phenomena, as well as the extension to multiple velocities are possible and proposed in complementary works.