that sounds really tiny.
The fiber pots can dry out very quickly, reducing the amount of useful moist soil in the pot. I would bottom water enough to keep the fiber visibly moist and dark, not light tan and dry. (I admit, I don;t have a lot of experience with these, for the reason of it being too dry.)
Are they under lights?
Celosia is generally easy. If you get them to bloom, and save at least one good flowerhead all the way to fall, you'll have hundreds of seeds for next year.
