The Tiger Barb can be a bit of a nasty fish, as you say aggressive. You really have to keep a large school of them to try and confine the nipping within the group, and have it spread out enough to not cause issues to any individual fish. You should have 8 minimum of them, and any other fish can not be slow/sedate and can not have long fins.
If you go with a Rainbow Shark you will have to avoid other bottom fish. You might be able to have some loaches, but that's it. The Red Tailed Shark is even more restrictive in what can be kept with it, no bottom fish at all with them.
The Clown Loach is a schooling fish, and gets large. You really need a 6+ foot tank for them.
There are profiles for many of the fish here on the site, the link to them is at the top of the page on the blue bar, second from the left. If you type out their name in a forum post, like you see in this one, the forum will automatically shade it and link to the profile so you can just click on the name. The profiles give tons of information, including what they can be kept with and the water parameters they need.