For a homogeneous and the isotropic universe, the spacetime metric $ds^2$ is given by the FRW form in comoving coordinates: $$ds^2=dt^2-a^2(t)\Big[\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2\theta d\phi^2)\Big].$$ It determines the LHS of the Einstein's field equation.
In Kolb and Turner's Cosmology book, it is said that to be compatible with the symmetries of the metric $ds^2$, the stress-energy tensor $T_{\mu\nu}$ should be diagonal and by isotropy, the spatial components must be equal.
Questions
How can I see that the symmetries of $g_{\mu\nu}$ must be present in that of $T_{\mu\nu}$? It is not obvious to me.
How do the symmetries dictate that the tensor $T_{\mu\nu}$ is diagonal? All I know is that if a Cartesian tensor $T_{ij}$ is invariant under rotation it will satisfy $$T^\prime_{ij}=R_{ik}T_{kl}(R^{-1})_{lj}=T_{ij}$$ where $R$ is the $3\times 3$ rotation matrix.