Previously i choose one that is about same size as my existing one and place my new one inside a beta box.
Initially my existing clown constantly checking out the new clown inside the beta box, after a few weeks i begin to see my old clown exhibiting some submission position (Swimming sideways showing belly up) which makes me think it's a good opportunity to try and release the new one together.
After one hour, i can see them swimming together happily. But that's just my experience not sure will it works for your case.