Yes, that would be fine to use. As long as the Roebic drain cleaner says 100% Sodium Hydroxide, you're good to go.
Why do you need to age 4-6 weeks? This allows the excess moisture to evaporate from the soap, making it harder (i.e. it lasts longer) and to lather better. HP soaps are safe to use right after un-molding, but so are CP soaps if they go through gelling stage & after they cool off. If you prevent or retard gelling a CP soap, then it may take up to a week for the soap to completely saponify.
I don't know if it will fit in your size crock. Any recipe smaller than that, I would definitely switch to weighing in grams instead of ounces.