isomorphic substitution in kaolinite?

You are correct about the structure of these two secondary phyllosilicate clay minerals. Kaolinite being a 1:1 phyllosilicate, where the layers are tightly held together by H bonding. Montmorillonite, a 2:1 phyllosilicate clay mineral, with shrink-swell properties.

Isomorphic substitution is a separate concept; the process by which one element of similar size fills the position of another within the crystal structure of a mineral. Commonly Fe²⁺ or Mg²⁺ replacing Al³⁺ in the octahedral layer of Al³⁺ replacing Si⁴⁺ in the tetrahedral layer. This substitution results in a net negative charge. Montmorillonite has a high degree of isomorphic substitution, while Kaolinite has very low isomorphic substitution.