I think you mean CLD (Constrained Layer Damping), not CLS. CLS was a great ESL made by Martin-Logan.
Mijo, The dogma would say you ought not to put any material between two layers where you want the impedance to energy sharing to be minimal. (IOW, you want to maximize the contact areas between the two materials to achieve effective CLD.) I refer to your warning to use felt between oak and metal. The felt might work to avoid staining of the steel, but it defeats the purpose of the layering. Similarly, the old practice of using a rubber profile between TT chassis and wood plinth is now frowned upon. Jean Nantais is among those who pointed this out.