There would be no problem with 8 labs in that tank, however good filtration and upkeep are a must, as mentioned. 5 or 6 might be a better number for the average fish keeper. I always start off with more juveniles to ensure you don't end up with more males then females, so I'd get 10 or 12 with the intention of keeping 8, or 8 or 10 with the intention of keeping 6.
I would not, however, subject the kribs to that kind of fate. Labs won't respect the kribs territory, and with all the labs swimming around they won't be able to do anything about it.