^Yep.
Interesting dynamics those Tiger Barbs.
Put them in with aggressive fish in a small group and they'll often get themselves killed by testing out the pecking order. You end of with the opposite of what is commonly expected, with the weak timid ones surviving and the dominant ones getting picked off. Give them a large enough school and they don't pick a fight with the other fish. But I'm pretty sure the Mbuna would be perfectly willing to bring the fight to them.
Either way you end up with a time-bomb since even if you do get a large enough school that doesn't get knocked out by the Mbuna, it is just a matter of time till old age (they don't live extremely long) starts to widle down the school and then they start looking at the other fish (which will kill them) to test out pecking order. Best not to do it.