They are definitely better left outside. But I'd wait until the cold weather starts & the slugs have gone to sleep. If they're not hardend off, place outside in a slug free area or on a shelf.
I leave them in pots, normally 6" or bigger untill the spring. Bury the pot in the ground to stop the wind knocking it over.
Plant in its final place around Easter, when the soil has warmed up a bit, otherwise it will outgrow the pot.