Watering daily in arid climate?

Once your plants droop, their health is suffering. Watering every day is fine if the soil is drying out fast. 100-250 ml at a time may be necessary and will not be too much, your seedlings will cope just fine, especially if you are growing in coco coir. Deep watering is far better than small, shallow watering (50 ml).

Only water when they droop and get that humidity up ASAP as that's bad for plants

Hi yes you can but be careful to not over watering your plants, 20% rh it is to low , try to keep the soil always a bit wet and try to low down the temperature 👌🍀