To answer your first question:
Particles and fields are separate. Particles are the irreducible excitations of fields. You can only get particles after quantizing fields.
However, you might often see people using particles without quantization of fields (in classical mechanics and GTR). You must understand that these are approximate models obtained by assuming the energy density of fields is concentrated in point like particles.
At the heart of non-quantized physics, we have continuous material fields for photons, electrons, quarks etc. These fields are (most generally tensor fields) of the form $${{\psi}_{(i)}}^{s...u}_{e....g}(x,y,z,t)$$ $(i)$ denotes the type of field (like photon, higgs boson etc.)These include scalars, vectors, co-vectors, spinors, etc. The Lagrangian density $L$ is usually function of the components of these various fields and the metric tensor. One needs to rely on observation and other considerations (like gauge symmetries) to construct a covariant (value always fixed at an event) Lagrangian. So the 'configuration' you speak of depends on all these factors.
Your second question is actually far more interesting. There are 2 SE tensors commonly used used.They differ by the divergence of an antisymmetric tensor. This paper: http://authors.library.caltech.edu/19366/1/GoMa1992.pdf discusses this in detail.
The first is the canonical SE tensor derived as a conserved current using Noether's theorem from the space-time translational invariance of the Lagrangian.
The second type of tensor is derived from considerations of diffeomorphic invariance of the action. It is called Belinfante - Rosenfeld SE tensor. A diffeomorphism is a very sophisticated and generalized notion of a translation. Let vector field $X$ be a generator of general diffeomorphism ${\phi}^*$ and volume of integration is $F$. $X$ vanishes outside $F$. So we have
$${\int}_{F}L{\eta}-{\phi}^*(L{\eta}) = 0$$
Thus
$${\int}_{F}D_{X}(L{\eta})=0$$ where ${\eta}$ is the volume form (I have suppressed the factor of 1/4!)
Expanding the RHS, we get:
$${\int}_{F}D_{X}(L{\eta}) = {\int}_{F}[(\frac{{\partial}L}{{\partial}{{{\psi}_{(i)}}^{s...u}_{e....g}}}-(\frac{{\partial}L}{{\partial}{{{\psi}_{(i)}}^{s...u}_{e....g;c}}})_{;c})D_{X}{{{\psi}_{(i)}}^{s...u}_{e....g}} + \frac{1}{2}T^{ab}D_{X}g_{ab}]{\eta}=0 $$
As you can see the first term is the Euler-Equation which is equal to zero for every field component, so each term in the first part of the integral vanishes.
Now a basic result that can be directly inferred from the definition of a diffeomorphism is $$D_{X}g_{ab}= X_{a;b} + X_{b;a}$$
Substituting this in the above formula
$${\int}_{F}D_{X}(L{\eta}) = {\int}_{F}((T^{ab}X_a)_{;b}-(T^{ab}_{;b})X_a){\eta}=0 $$
The first term can be transformed into a surface integral on the boundary of $F$ and vanishes as $X$ vanishes on the boundary of $F$. This leaves us with $${\int}_{F}D_{X}(L{\eta}) = {\int}_{F}-(T^{ab}_{;b})X_a{\eta}=0 $$
Now the above must always be true for any arbitrary $X$, this is only possible if $(T^{ab}_{;b})=0$ .
This tensor is always symmetric and gauge invariant, so it is far more useful in GTR than the canonical tensor. Refer to the paper linked above to know more details about the subtle differences between the two.
References:
Chapter 3, 'Large Scale Structure of Space-Time' by Hawking and Ellis
It makes sense to prove this using incopressibility and the equations of motion for the fluid. Start with:
$\nabla _{\mu } T^{\mu\nu}=(\rho +p)\nabla _{\mu }(U^{\mu}U^{\nu})+p\nabla _{\mu }g^{\mu\nu} $
The second term vanishes for the Levi-Civita connection (metric compatibility).
The first term is :
$\nabla _{\mu }(U^{\mu}U^{\nu})=U^{\nu}\nabla_{\mu}U^{\mu} +U^{\mu}\nabla_{\mu}U^{\nu}$
If the fluid is incompressible: $\nabla_{\mu}U^{\mu}=0$
If the fluid flows on geodesics: $U^{\mu}\nabla_{\mu}U^{\nu}=(\nabla _{U}U)^{\nu}=0$
So those 2 conditions imply conservation of energy-momenum.
Edit: As pointed out by OP, this treatment is partial and assumes both incompressibility and constant pressure. In the full treatment, one take the equation $\nabla _{\mu } T^{\mu\nu}=0$ , substitutes T for a perfect fluid as above, and projects the equation on different directions to get the 2 equations:
$U^\mu\partial _\mu \rho =-(\rho +p)\nabla_{\mu}U^{\mu} $
$(\rho +p)U^{\mu}\nabla_{\mu}U_{\nu}=-\nabla_{\nu}p-U_{\nu }U^{\mu }\nabla_{\mu}p $
The first equation is a conservation equation $\rho$, and the second is a conservation equation for the momentum.
Reference: Relativistic Fluid Dynamics notes by Jason Olsthoorn
Sections 3.1-3.3
http://mathreview.uwaterloo.ca/archive/voli/2/olsthoorn.pdf
Best Answer
An action principle for general relativistic perfect fluids is given in Section 5 of
Marsden, J. E., Montgomery, R., Morrison, P. J., & Thompson, W. B. (1986). Covariant Poisson brackets for classical fields. Annals of Physics, 169(1), 29-47.