The Apogee SP-110 will work fine with the Mayfly. Just connect the signal wires to AA0 and AA1 (the aux analog inputs) and do a differential measurement. You’ll need to use the Adafruit_ADS1x15 library and look at the example for differential measurements.
We always use the amplified output versions of Apogee’s solar sensors instead of the self-powered ones because they have much better sensitivity. There’s an order of magnitude improvement in the sensitivity by using the amplified ones, and it results in a much cleaner signal too. The SP-212 is the amplified (2.5v) equivalent of the SP-110, so I would recommend that instead if you haven’t already purchased the sensor.