The OS determines whats on the SSD. its completely transparent to the user.
If you want to use an SSD for the AE cache there is no way to set unless you have a discreet SSD.
i work at a place where they put an SSD in an external TB case and have it mounted to the back of the imac and thunderbolt connected, so thats an option as well.
If you have an extra system that you want to see how it may work there apparently is a way from the terminal to make your HD and SSD one volume and OSX will manage it as a fusion drive. i have it tried it so ymmv...