What we're learning: 

this isn't necesarily a pico problem (haven't 100% cleared this) 

linking works on pico when there isn't a class used 

which means my research really has nothing to do with pico, 
and everything to do with building cmake and accessing external classes

ATTENTION:

you won't be able to get classes to talk to each other with pico, until you also add the .cpp file to the cmakelists "add executable" list - see petRock tests for example

add_executable(projectName
    externalClass1.cpp
    externalClass2.cpp
)