My main question is, am I able to use a solar panel to only allow power flow when it is dark outside?/When the Solar panel isn't receiving any light? What would the circuit for that look like? In the same way a garden light works?
This is a simple PNP circuit with a light dependent resistor. When the sun goes down the resistance of the LDR increases and the voltage drop across the LDR forward biases the PNP transistor and illuminating the LED or whatever you like within the operating range of the transistor.
This should be a good start
Here is another circuit that I found at http://www.talkingelectronics.com/projects/SolarLight/SolarLight.html