Arturo, you obviously have a much different climate than I do! (Picture about 400 miles north of Ann Arbor...)
For the celeriac, yes, I think probably 8-10 inches spacing would be right. That always seems like so far apart when the plants are little... And yes, it does like moist soil, so a mulch to help conserve the moisture can help.
I think I might try growing celery in my greenhouse next summer, in a large planter -- it definitely would be easier to control the conditions! (and I can't give up on growing it until I get at least one decent crop...
)
Happy gardening! in just a couple of months I can get busy starting some seeds under lights indoors for next summer's garden...
