ChilDawg: I don't think that you hijacked the thread, and agree with your thoughts about the 125g mentioned by stik. Although that tank may be interesting now (I am assuming that the fish are all juveniles), it is going to turn into a holocaust when some of the inhabitants start putting on some size. Many will get eaten, and then others will die from poor water quality as the survivors grow to their natural size. You can't even keep one pacu in a 125 (too small for them to turn around in), much less with all of those other sizeable fish (ghost knife, channel cat, bala sharks, and tinfoil barbs, to name just a few).
To go back to the original question, a 100 gallon tank with 20 2-inch tiger barbs should already be a spectacular sight. If I were going to add to that mix, I'd put in some loaches (not clowns, but only because their coloring is too similar), a smaller school of larger barbs, and perhaps a school of the larger hatchetfish. You could also go for a pair of the smaller geophagus or gymnogeophagus cichlids, as they are relatively mild-mannered and should not bother the other fish. Half of the fish I recommended are from SE Asia, and the rest from S. America...so much for the biotope tank, I guess.