40 seems a little tight for even Blue acara. I had a breeding pair that were 5-6" long. They are a pretty active species and I would think that they would appreciate a bit more swimming space. Kribensis or Apistgramma species would be pretty and better suit your tank. Cory cats and tetra species can also be housed with kribs or apisto.
I agree and think these are the most suitable of all the fish mentioned so far.
I would o ly consider true dwarf species from west Africa or south America for a tank that small.
The only c/a fish I would consider are rainbow cichlids but I think they could also do with a little more space.
Don't think they have been mentioned but look at a cousin of the Convict the Honduran Red Point.
Amatitlania sp. “Honduran Red Point”
They stay smaller than your normal Convict males around 4in, have a lot smaller spawns if you let them breed, around 50 or so instead of hundreds.
Mine didn't mess with plants unless they decided to spawn right next to it. They also really don't bother non cichlids. Mine have been housed with Tetras, Bushynose pleco, Roseline sharks, Plattys, Swordtails, various catfish and never had them hurt the other fish. They will defend fry, but only chase a short way then return quickly to the spawn.