So you feel it's safe to add, say one cory and some Safe Start now, and then maybe another in a week, etc?
Also, why wouldn't the tank cycle for the bioload of the one fish AND the bioload from the ammonia released from the water changes? Prime does say that it makes the ammonia available for the biofilter, and the frequent water changes have been going on since the tank was set up because of the fish being ill. So I would think the bacteria would have built up by now to accomodate the extra ammonia from the frequent water changes. Note that one the days I've added either filter squeezings or Safe Start I have waited 4 days to do any water change.