Yes, as everyone said, yellow labs are african and should not be kept with new worlds. If your water has a high pH then maybe consider getting africans instead. They would love the water as it comes and you wouldn't have to worry about messing with the pH (as it can run you into some big troubles). But this also depends on how high your pH is, because if its around 7.5 or lower than this is ok for the new worlds.
When you say 6 ft tank, this leaves a lot tank sizes to fill this in. What are its other dimensions or better yet, its volume. This will help determine compatibilty for other fish as some other new worlds grow upwards to the size of oscars and having too many of these could lead to serious tank health problems as well as your oscar possiblity getting beat up. Yes, I said, your oscar. They are quite timid, or i should say 'gentle giants' more than anything else. They are far from the most aggressive cichlid species and people think because they big, they can handle themselves. With faster moving or more aggressive fish, they can tend to get nervous and when startled will jolt in your tank and could possibly run into decorations or the glass (ask anyone who owns an oscar and they will tell you how clumbsy they can be).
The pleco too, will create more waste in your tank than it will to help keep it clean. This is the misconception that they are algae eaters and therefore can help keep the tank clean. Well, with any algae eater, ie plecos, snails, etc.... the more they eat, the more they poop. Soon your tank is overridden with poop. But it is still managible if you don't mind doing very thorough vacs of the substrate about every 5 to 7 days. Plus, some people believe keeping a plec with an oscar is bad just because they are both 'messy' fish and add alot of bioload to the tank and thereby increasing the amount of time for maintanence.
