Can I use the MiniRambo 12 volts or should it be a separate power source?
What you can or cannot do will depend on the specs of the solenoid. If you post a link to the solenoid your purchased, or better yet the datasheet, then forum members are going to be able to give you good advice on how the solenoid needs to be installed. It is possible, even likely, that you can drive it directly off the fan pins. The circuit provides for up to 5A for the heated bed, fans, and logic, and with no heated bed, you should have plenty of headroom to drive a solenoid. You may or may not have to upgrade your power supply for the MiniRambo.
As Jeff suggests, you can use the M106 and M107. Near as I can tell, there are only two fan ports on MiniRambo. They are labeled Fan1 and Fan2, so you may have to add a parameter to the g-code. Something like:
M106 P2 ; Turn on Fan2
M107 P2 ; Turn off Fan2
The first step will be to execute an M106 and checking the fan pins with a multimeter.
I’m curious about the solenoid. Other than moving faster, what doe sit provide that cannot be done just by moving the Z axis up and down?