I kept my Amphilophus Citrinellus pair in a 6ft 220G with 5 Melanotsemis Trifasciata and 8 Neetroplus Nematopus. Everything was fine until the Citrinellus spawned and then all of the Nematopus were a threat and chased / attacked relentlessly and no allowed to leave the rock work, to the point I had to remove them. The rainbowfish are not seen as a threat and left alone unless they get too close to the cloud of fry.
If you were adiment on adding something to help your red devil feel more comfortable instead of waiting I’d try a fast swimming, top dwelling fish such as tetras, Danio aequipinnatus, or Myles as stated above.
I have had good experience with non cichlid dithers bringing out shy fish. But personally I don’t think it is required and with more time your red devil should come around.