I had an EB Johanni in with my 5 Yellow Labs, etc and it was way too aggressive; it was nearly killing them, so I had to trade it in. I currently have a female Borleyi (Red Fin) in my 55, but it's going to get too big (8") for a 55, so I will have to trade it in as well. I also have a Featherfin Squeaker, but again having learned that they can get up to 8", I may have to trade that one up as well.
You really don't want to get any fish that will exceed 6" in a standard 55 gallon aquarium. Having to trade up fish that don't work out really stinks, so I would really do your homework before you decide on what to add. I'm actually trying to find suitable tank mates for my 5 Labs in my 55 as well, so I'll be interested in hearing what folks recommend to you.
I have been considering an Auratus myself. It's the right size, but I'm not 100% convinced that they are not too aggressive. They sure are pretty, though.
Some other unasked for advice
If you have multiple Yellow Labs you should consider 4 or more or the dominant male may be too aggressive with all others and stress them to death. When I had just 2 one was always beating up the other one. No problems at all with 5, though. They look much better in a group anyway.
If you have multiple Aceis they should be in groups of at least 3:1 F:M ratio or the males may be too aggressive with the females and stress them to death. I've had terrible luck keeping Aceis alive. I've had two batches and they were both wiped out by some strange blight. Some folks say they are too aggressive to go with Labs, but I'm not sure I buy that. I may give them a try again if I have the capacity to get the right ratio and number of them.
So, really, you could pick up 3 more Labs and 3 or 4 or more Acei. You may want to keep your Auratus solitary. It looks like they are very aggressive with each other. Whatever you choose, stick with Mbuna. Don't mix Mbuna, Peacocks, and/or Haps.
I've also had people recommend Red Zebras, but again, opinions vary on their compatibility with Labs.