Yes, if you are concerned about bringing nasties to the rest of your birds, the best plan is to enlist some professional help in testing for disease and a fecal float for worms. Sometimes birds have an immunity to whatever was living in the soil where they came from, but it will cause your birds to sicken. I only say this because I know you've been working hard at breeding combinations in your flock, and I would hate to have you jeopardize all of this.
If testing isn't an option, I would just take is slow and introduce a few birds at a time to him in a controlled location and watch how it goes. He might be just fine, and he looks like a nice maran rooster for all that he has likely been through.
