My understanding of the Falcon Heavy cross feed system is that the outer cores feed six of the nine engines directly on the center core. Thus, the cross feeds don't pump propellent and oxidizer into the center core tanks but rather feed six of the nine center core engines directly.
This is very similar to how the Space Shuttle worked. The Shuttle's cross feed between the External Tank and the Orbiter was 17" in diameter -- one for propellent, one for oxidizer. A project to reduce the size of the cross feed to 14" was cancelled. A video of the disconnect being tested can be seen here:.
How large are the cross feeds between the Falcon Heavy's outer cores and the inner core?
Are there four (2 for propellent, 2 for oxidizers)?
Has cross-feeding been done on any other vehicles other than the Shuttle and the Falcon Heavy?
Do the pumps reside on the center core?