1) yes, always use gpio number i.e. 21
2) adafruit mandated feed names to be small case and without space (single word). Reef/pi currently does not support multiple temp probe. So the feeds used currently are ato, heater , temperature , cooler , heater, channel-1, channel-2 etc
3) float switch should work just fine in place of photo sensor. You need the additional breakout board with photo sensor, it maps the sensor’ 4 pin configuration to 3 pin setup with appropriate resistors and diodes in between. It’s a simple circuit and the schematic available online (yay openhaEdward) , so if you want you can roll your own pretty easily
4) that’s a hard question to answer due to the fact that we are not settled on a fixed lighting . I have use kessil and the npn transistor route is enough for me. Since there is no common or open standard which all lighting manufacturer adhere to, I’m inclined to say that reef-pi will standardize on software /hardware that generates 5v pwm and also document handful of ways to convert that to 10v pwm (using npn transistors ) or 0-10 v analog (using lpf).
Does this answer your questions?