coopshoot,
First allow me to be the first to welcome you to a truly wonderful gun board. I think you, like so many others, will find this a warm and welcoming bunch of men and women with similar desires and attitudes with regard to firearms and enjoying the shooting experience.
As to your excellent question...
I can say with absolute certainty that ANY 2-3/4" commercially loaded shotshell that comes in a "low brass" configuration is TOTALLY safe to shoot. And "high'brass" 2-3/4" heavy field loads are perfectly safe as well, such as pheasant loads of #6, or 2-3/4" turkey loads of large shot (#2, #4, etc).
Beyond that, while I am almost absolutely certain they are safe, I would defer to the real experts and call O.F. Mossberg Customer Service and simply ask them about such things as 2-3/4" buckshot, or slugs, or even 3" loads. While you have them on the phone, get the representative's name and also ask about such things as steel shot for waterfowl. You may never buy that ammo, but it doesn't hurt to ask!
And additionally you might find there are answers to your questions by checking out the FAQs on the mossberg.com site. I know what's there but I want you to look as well rather than have the links posted or cut-and-paste quotes.... (wink).
But I'm glad you're here and I'm glad you are inquisitive. We hope you learn your new shotgun until it's like an extension of your body and we look forward to many more posts from you!