OK. I have four tiger barbs in my ten gallon tank. They can be agressive. I tried to put an angel fish in with them, but within an hour they began to nip it's fins, so I had to remove it. I have heard people say that platies can go with barbs, but I haven't tried this myself.
You definately want to avoid long finned fish, such as bettas, as their fins will be constantly picked on (or should I say off?
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I have also heard that Zebra Danios do ok, since they are fast swimmers, but again, I've not tried this with mine.
In case you were wondering why I've heard so much, I've started topics like yours before. I didn't get any suggestions that I really wanted to try, so my barbs are still by themselves, except for my spotted ralphael catfish, which will work with pretty much any fish.
I guess the only thing I could suggest that I personally know will work is bottom feeders. There are nice Plecos and catfish that the barbs will leave alone. I like my catfish, but he's nocturnal, so I hardly ever see him. There are more visiable species. Try a cory or two.
You could also try any of the sharks (bala, red finned, etc). They can get big, but I don't think the barbs will pick on them too much.