I would never requeen a swarm from an unknown source unless they were mean. That is the best chance you will ever have of getting a mite resistant strain of bee. Watch the swarm's progress and if there is a reason to requeen, do so. Otherwise, leave it alone. If it is loaded with mites, if it doesn't produce well, if the queen doesn't lay a decent pattern, if they are mean, requeen. Otherwise, keep the feral genetics.