Basically I agree with what everybody else has said. Tiger barbs will not bother catfish or loaches, but like angel said, they will compete for food. However, you can keep regular gouramis (blue, pearl, etc) with them, but I don't recommend the dwarfs. The regular gouramis are agressive enough to stand their own ground.
In a 30 gal you can keep mollies & platies with them, tetras like neons and head and tail light, danios, and hatchetfish. I suggest you make it a barb tank. Add some rosy barbs or cherry barbs, or you could also do green or albino tiger barbs.