These rock... Mine is peaceful, but mix at your own risk. I picked mine at the LFS by asking the keeper to feed the tank and then choosing the tiger that was most interested in food and least interested in other fish. I have 2.5" zodiac loaches that zip around with the tiger and cuddle up. The primary obstacle to owning aggressive loaches is having enough nooks and crannies that they can easily claim a territory that is not immediately encroached upon by other loaches. Nonwithstanding, given the shape of their mouths and the fact that they only have pharyngeal teeth (that is teeth at the back of their throats), they can't really do major damage to other fish. Defensively, many species of loaches have sub-ocular spines, but those are rarely used in aggression.
For anyone looking for these, we are seeing Tiger Loaches come in on a regular basis right now. Our store is seeing an influx in uncommon loaches, and you can probably get them if you ask for them, if your LFS doesn't stock them. We are currently seeing Syncrossus Helodes and Syncrossus Hymenophysa. In addition to the Tiger Loaches, we have seen some of the Schistura species available as well as Sinibotia Robusta, Botia Striata, Botia Dario, and Botia Histrionica. I wouldn't consider these to be rare, but many LFS in my area don't stock them on a regular basis. If you don't see them at the better stores in your area, ask for them.
Also, this is an awesome source for loaches. I plan to get some Tenasserim river loaches next summer if they are still available. The price has come down considerably.