I had a solo clown for about a year that's 3" long. I added another one that was only about 1/4 of the size and for the first day or two the big one would act aggresive towards it, but now they dont separate from each other.
When adding another clown into a tank that already has a single clown, there is usually always some "squabbling"...its nature, its the larger one stomping its dominance on the other and certainly nothing to worry about..