  Using the Lieb-Mattis ordering theorem of electronic energy levels, we
identify the Hilbert space of the low energy sector of U($N$) quantum
Hall/Heisenberg ferromagnets at filling factor $M$ for $L$ Landau/lattice sites
with the carrier space of irreducible representations of U($N$) described by
rectangular Young tableaux of $M$ rows and $L$ columns, and associated with
Grassmannian phase spaces U($N$)/U($M$)$\times$U($N-M$). We embed this
$N$-component fermion mixture in Fock space through a Schwinger-Jordan (boson
and fermion) representation of U($N$)-spin operators. We provide different
realizations of basis vectors using Young diagrams, Gelfand-Tsetlin patterns
and Fock states (for an electron/flux occupation number in the
fermionic/bosonic representation). U($N$)-spin operator matrix elements in the
Gelfand-Tsetlin basis are explicitly given. Coherent state excitations above
the ground state are computed and labeled by complex $(N-M)\times M$ matrix
points $Z$ on the Grassmannian phase space. They adopt the form of a U($N$)
displaced/rotated highest-weight vector, or a multinomial Bose-Einstein
condensate in the flux occupation number representation. Replacing U($N$)-spin
operators by their expectation values in a Grassmannian coherent state allows
for a semi-classical treatment of the low energy (long wavelength)
U($N$)-spin-wave coherent excitations (skyrmions) of U($N$) quantum Hall
ferromagnets in terms of Grasmannian nonlinear sigma models.
