My Jacks are alone in a tank as they are a pair. They can get quite evil when they spawn. But tankmates are not impossible at all. Most fish will tolerate Jacks and vice versa. Especially if you have the lone Jack Dempsey (is yours female or male) The only time I really saw problems with tankmates when it came to mine was when they would spawn. My 3 inch female could put an oscar on the floor. I also tried to combine tank and found that while my female Jack and female Green Terror got along just fine, the males sparred for days. With Cichlids, its reallya guessing game. It is hard to say if they will get along. I always figured cichlids choose their tankmates, not us.
I would stick with trying cichlids that are along the same size as your Jack. Have a backup plan in case it doesnt work.
I would stick with trying cichlids that are along the same size as your Jack. Have a backup plan in case it doesnt work.