That's a delightful tank!
My own long-ago experience with Keyholes was that they are indeed the quiet, shy fish most consider them to be, but once a pair forms in a community tank and breeding begins, they turn into...cichlids! They protected their nests pretty aggressively. My group consisted of 3M and 3F and once they reached maturity there was always a bit of chasing and posturing between them, ond once eggs appeared it was necessary to remove the non-breeders unless the tank was at least 4 feet long. Males were easily 4 inches long, females a bit less.
You have a lovely community tank now. I think that adding more Keyholes is very likely to turn it into a totally different type of tank: less peaceful, less attractive, higher stress, higher maintenance and in some ways more interesting.
If it were mine, I would leave it as is. The single Keyhole will become less shy as time passes and will grow into a discreetly beautiful centerpiece who won't bother anybody and won't disturb those lovely plants.