Size difference is the key. When making a pair rock rearrangements are not really required. All you need to make sure off is that the new fish is of the opposite sex.
If your current fish is female, you can just add a larger male and they should get along just fine.
If your current fish is male you may need to catch him and keep him confined for a few days while the smaller female gets to know the tank. After 2, 3 days you can add the male in a clear container and see how they behave. The female should show submissiveness by forming a "S" shape with her body, tilts a bit to the side and trying to look as small as possible. The male should after that display not show any aggression towards her. If this is the case you can let them together.
If the female tries to look as large as possible by putting all fins up you may have two males or two equally sized females.