A Southern magnolia is a very large tree. I don't think it would be very happy in a pot. It's kinda like putting an oak in a pot. You can do it as a seedling, sapling, but eventually you should put it in the ground. However, it can't hurt to try it and it may live several years. Not sure how slow- or fast-growing it is.

They aren't real fast growers but they can shoot up more than a foot over a season, sometimes even more. I have to agree that it won't be happy long in a pot. They have a fairly extensive root system too.
Tally Ho!

they will do fine in a pot. i've got two i'm training as bonsai (from seed). just keep them under control. repot into larger pots or trim the roots and manage the top growth as well.

Will they flower in a pot?

The southern magnolia (at least ours) has to be a pretty large and older tree before it blooms, even when planted in the ground. The one on our lot has been there for over twenty years before it bloomed, so don't hold your breath on a potted one blooming. On the other hand, Elberta Alabama, has the Little Gem Magnolia planted in large pots along their main street. They are looking good, but I wonder if even they will be long lived in pots?