We have had the best luck intro-ing young chins, especially babies, to older chins. That said, we have intro'd plenty of older chins to older chins and have had successful pairs. However, the easiest intro's we've had have been intro-ing a baby to an older chin. Those seem to typically go off without any problems.
If you don't want to get a baby, then maybe try a chin that's a bit older, but not yet an adult. In our experience, the older they get without having a cagemate, the less thrilled they are to have one later in life. So, if you don't want a baby, maybe get one that's several months old, but still younger than a year, so it's not quite an adult yet.
Best of luck!