I put a silver arowana in my 150g, then a few weeks later added a pair of yellow labs, among other mbuna. The yellow labs are now about 3" and 4" and the arowana is about 12". Neither of the labs have been eaten, nor has the arowana tried to, at least when I have been watching. Not to mention I don't have as many hiding places as I'd like for mbuna. I would bet that this is the exception rather than the norm. However, I have no doubt the arowana will eat the labs when he gets bigger- but he'll be moving to a 500g pond without the labs in a month or two. I'd try it in your situation, as long as you're not particularly attached to the labs and your financial situation can take the possible hit, which will be substantial with that many labs. Keep in mind though, it's more than likely that the aro will eat at least a few, in which case you should remove the labs (maybe make the old 6 foot a mbuna tank?). Also, I highly doubt the labs will gang up on the aro. Plus you have the bonus of the 8ft long big tank, that will definitely help the labs hide.
Edit: My silver aro also eats the occasional feeder when he steals them from a needlefish, but this doesn't seem to boost his aggression.
And f22; water parameters as far as pH goes, these fish have been tank bred for generation upon generation, which makes it highly likely that both the labs and aro won't be as picky about the pH (assuming neither are wild-caught). I wouldn't doubt that keeping the pH in between the preference of the aro and the labs won't harm either of them.
