Silver Dollars max at 6", should be kept in shoals of atleast five (5) fish and like soft acidic water and need a minimum 35gal tank for five (5) fish.
Tiger Barbs ( sometimes refered to as those little fin-nipping *#!^*#'s)also require atleast five (5) fish, preferably more (6-8+) so as to spread out aggression particularly when tankmates with other fish.
Also take into consideration this statement from AC's own SD profile,
When feeding your fishes it is recommended that you make sure your Silver Dollars are getting their share of food as they are often "out-matched" at feeding time by more aggressive feeders such as large Barbs and Tetras.
You could keep them together, but IMO, not in a 29gal.
Your Africans absolutely love hard alkaline water, granted they like most fish can adapt to just about any water condition, they will thrive better in their preferred water... Liquid rock

.
SD's = Soft & Acidic, Africans = Hard & alkaline...
I wouldn't keep'em together, its just not good fishkeeping/husbandry.
The keeping of S. Americans with Africans is the fishkeeping equivalent of religion and politics...
Sure to start arguements!
Just my $0.02