Typically cooling fans should not be used on the first layer as this is the time when you want to ensure as much adhesion as humanly possible.
The old mantra was always that PLA uses cooling and fans and that the more the better and the ABS didn’t use fans at all as you wanted to ensure ABS called down slowly as possible due to its fragile nature while cooling.
However modern printers tend to have full enclosures where we can now guarantee that the interior temperature remains high at least over 40°.
ABS can actually benefit quite well from cooling, maybe not hundred percent fan from the start of the print however 50% after the first six layers also and you can read tighten up smaller sections of your model.
PLA you’d hundred percent pretty much after the first couple of layers and get massive benefit from that with your enclosure fully opened.
For ABS you’d only use 100% for bridges and overhangs if your slicer allows.
Typically extruders only have a single cooler if any however it is usually fairly painless to add a second fan usually on the other side.
If you have a dual extruder 3-D printer and you find yourself only using one extruder, it’s a very convenient way of adding a second cooling system as you can remove an extruder re-purpose the fan and simply add a second nozzle on the carriage alongside the remaining extruder.
Cooling is an area of 3-D printing which is constantly evolving and adapting, will be sure to keep you updated with our findings.