I agree with ehh, he's been in there for so long by himself adding anything else would be a real challenge. I keep my breeding pair of Carpintis in a 120 gallon tank (60" x 18") with a handful of Giant Danios.
You could always try a school of fast moving fish. Get some giant danios. It will be easier to sneak some new tank mates in while the carpintis is distracted with dithers.
He has been in with bristlenose catfish and kuhli loaches the whole time and never touched any of them though and both the catfish and loaches steal his food all the time with no issues so I just assumed that he may be more tolerant of smaller fish and only attack other fish of equivalent size which I have tried in the past.
Just for my clarification , what makes/made you think that the Carpintes has mellowed down.
Also , do you have any backup plan/extra tank incase one of them gets injured.