Pigs of different ages generally mix very well, and if you added a new weaner it would be around 8 weeks and your existing one is around 12 weeks, so there will be a good difference in size. Whilst there may be some initial pushing, they should quickly establish order.
They may take a day or two to integrate fully, so allow two sleeping areas for the first night or two - you should be able to improvise a second shelter - few bales of straw with a tarpaulin roof so that new one does not sleep outside.
We regularly mix pigs of all different ages in our fattening gorup, and harmony is soon restored.
Yesterday we put a 12 week in with two 8 week olds, and this evening they are all sleeping together.
And yes you should absolutely get another, pigs alone are miserable.