Son and Son Montuno are basically the same thing. The main difference is that the Son does not have two distinct sections.
Like most modern music, there are many exceptions and sometimes there is no clear defining style. There are still rules and standard patterns though. If you learn these basic standard patterns that the Cuban tres plays, you will be able to mix up the styles and create something unique.
Just remember, on the Cuban tres, like any instrument, make sure you know and understand the rules before you break them! That means listen to a lot of Cuban music, especially music that features the Cuban tres.