A system for sampling and/or conditioning a process gas such as natural gas or the like utilizing two or more modular components, each having unique conditioning or monitoring features or the like, which components are formed to be slidingly received in a receiver so as to be stacked one upon the other for sealed engagement, forming a serial flow-through passage to provide conditioning, monitoring, or other features as the gas flows therethrough. An embodiment of the present invention is designed to receive multiple conditioning components, each of which may have diverse functionality such as, for example, one or more stages of pressure reduction, monitoring, particulate and/or liquid droplet filtering, etc. The invention is designed for easy configuration customization so as to provide a readily customizable solution for each application, with an embodiment which may be incorporated into a probe or the like or at its tip, and thereby operate at the prevailing pressure and temperature of the process gas.