If adding one later did have to happen, he's gotta be larger than the current inhabitants IME. I added a yellow (slightly larger) with a purple and a bristletooth. One or two days of fighting with the purple (since they are of the same family), and the three of'm hung out all the time as BFFs.
Be aware that the yellow and the sailfin are in the same family, and if there is any agression, it'll be between those two. Gets some algae and clips for them. Also, try soaking your food in Garlic Guard or a reasonable facsimile to keep their immune systems up while they adjust to the stress of a new tank and each other.
Best of luck! Great looking fish.
"Life may not be the party we hoped for but while we are here we might as well dance!"