5 gallon works if it's actually 5 and not one of those "3 gallon" boxes that lie on the label. Filter matters more than people think — sponge filter is fine, just don't blast the betta with flow.
I'd cycle it. Even a betta hates ammonia. Fish-in with daily Prime and testing works if you're impatient, but since you already test for reef stuff just do the boring fishless week.
