I agree, as a former boarder, that there are a lot of boarders who feed on the drama - and there's always some type of drama in a barn. If you apologize to the new boarders for letting it slip out in the newsletter and causing problems for them, they will have to respect you for that. You could maybe give them a discount privately on their first month's board or throw in something extra for them? Let's face it, most people today don't have the integrity to own up to an error in judgment (such as saying, "I'm sorry, I didn't realize how that sounded in print; it's a lesson learned that people gossip in the horsey community...") and it takes the wind out of others' sails when you own up that you would do it differently the next time.
Do this face to face - it's too easy to make mistakes when something is put in writing, it's misunderstood or picked apart word by word. (Just like the newsletter.)
I don't know that I agree that an apology is necesary to the former barn owners...IMHO that depends on your community and how connected the barns are in your area, and how much damage the newsletter did to your own business and reputation. I think that if the former barn owners are mad, their action will be to go to a lawyer whether or not they get an apology from you...and anything you say can be used against you.
And I agree with the other posters that your grumpy boarder needs to be talked with, to let them express whatever burr is under their saddle, so to speak. You may or may not want to try to fix the problem, but it's fair to ask them to bring their issues directly to you instead of spreading innuendo and bad mouthing you to others. I think if I ever were a barn owner I'd make up a 'code of conduct' for boarders, to include that type of thing! Tell me first, or hold your tongue, so to speak!