I think that is a fine size for one, maybe to. I sometimes have to cram two moms into my little cat litter pan tubs if one mom isn't doing a great job on her own. You just have to clean the tub more, not a big deal. I think my litter pans are about the same size as a 10 gallon.
As for the mixing tub question, I think my cement mixing tubs are about that size. I use them as breeding and grow out tubs. I like to keep 4 adults per tub. That gives them enough space and bedding that they can go a week before they really need cleaned.