It is quite possible that they will start trying to determine who is dominant the moment the new fish is added. Sort of like the new kid in class. This does not mean they are paired yet.
First they have to determine who is the dominantfish, large size difference is a good first step but may not be sufficient. For example, if your smaller fish has been in your tank for a long time but is only small because you haven't been feeding it enough and the bigger fish is coming from a well fed tank with a lot of clowns in it you may well have a case where the smaller fish is going to be dominant.
Once the dominance has been sorted out the dominant fish will become female, if they are not yet sexually mature fish this may take a while, if they are mature but just not yet sexually differentiated it may only take a month or two. Or, they may already be sexually mature and differentiated in which case the next phase starts.
Once they are both mature and sexually differentiated (you have one male and one female) then the female will set up home of some sort and the male will have to earn acceptance by the female. This may happen rather quickly or it may never happen, it's up to the whim of the female.
Keep feeding them A LOT and keep up the water changes. These 2 things will make for the healthiest and longest lifes possible for them.
