I've heard of tiger barbs being dither fish for africans before....don't put them in if you want them to stay all pretty and nice. Chances are once the mbuna grow the tigers will become the focus of their aggression and become ragged.
I think just for dithers, the buenos aires tetra might work better theyre faster and i think tougher than tiger barbs. keep in mind that tiger barbs are rather colorful. the tetras, however; wont distract from the beauty& color of the cichlids.
I've had BA tetras with my mbuna/haps before and they nipped the extensions off all my most attractive fish.
And unless the tiger barbs were very small I don't see why the mbuna wouldn't eventually get used to them and leave them be.
For the most part, the point of overstocking with mbuna is so they act as their own dithers.