Acei are really wonderful fish, they are on the more peaceful side of the mbunas, tend to swim around the top and make for an impressive fish when full grown. I did keep them in my 72g (4ftx1.5ft) tank for awhile but they really needed a bigger tank by the time I sold them being that my male was nearing 7" and they are rather active. Since your tank isn't as deep I don't think they'd be happy there long term, just means you'll need a bigger tank eventually!
If you are trying to stay with the more peaceful species...you picked the wrong group of fish.
But seriously rusties and acei are within the labs temperament. Red zebras tend to be ruthless monsters. I didn't want to believe it when I got my OB red zebras (avatar) but everyone was right and I had to get rid of them because they just started tearing each other apart, literally! Demasoni could be a beautiful display with your labs but they are mean to each other and you have to get at least 12 adults to spread the aggression or you'll end up with 1. That is something I wanted to do myself actually because it does make a beautiful tank with minimal effort too.
Also, adding new guys to older guys can be VERY tricky. Labs MAY not be too bad but make sure the new guys aren't too much smaller then the old guys and re-arrange the tank when you add the new ones. Since you already have fish in the tank and can't add them all at once add the least aggressive species first and try to add them in the biggest groups you can at a time so they can spread the aggression out. Good luck!
