I'd say keep em in a different bowl for at least a week before adding them with your betta. If you want to take your chances go ahead, most people do. My mother lost her 4 year old koi this year because my sister added a goldfish right away and it brought some bacteria from the tanks at walmart. I personally wait until they go a week without any dying before I add fish with my goldfish because I've had her for three years and use her for breeding so she is quite important to me.