The roots will be fine. I water once a day, I could water once every few days if I felt like it. Check out my grow in the link in my signature. These are in tiny rockwool croûtons in 5 1/2 inch pots. I usually grow in 6 inch rockwool cubes. You can place 4 in a cubic foot. So in other words right next to each other. I have done it with a coco mat underneath and without. It did not make a difference. I usually veg 1-2 weeks. Redwhit is right, it will be kind of a pain to hand water. You can build a system really easy. Why aren't you?
You can make a gravity one with a bucket and a tube or a pump one. Check out my grow for some ideas. peace