Structures of gels and clusters of disordered rods and fibers are described by scaling relations for a homogeneous random fiber network and a heterogeneous (that is, fractal) fiber structure. Both models relate the gel density to the particle shape. It is shown that gel densities for attractive colloid rods (iron hydroxide, clay minerals) are often too low to fill space with a homogeneous microstructure. Space filling with heterogeneous rod clusters allows for a very low solid content due to a combination of high particle aspect ratios and low fractal dimensions. This view is supported by reanalysis of early simulations of Vold (J. Phys. Chem. 1959, 63, 1608) of ballistic rod deposition, and yield stress measurements on boehmite-rod suspensions in an accompanying paper (Wierenga et al. Langmuir 1998, 14, 0000). Attention is also given to rapid rod coagulation and retarding factors, such as polydispersity in attraction strength, which may strongly affect gelation kinetics.