Red Tailed Sharks are beautiful fish but need large bottom swimming space. They can also be aggressive towards fish of the same species or to other bottom feeders. I however have not had any problems between mine and my sailfin pleco.
A beautiful fish. They need to be able to establish their own territory- provide them with a cave. IME they often will get along with loaches, and may be willing to share their cave, though they can be very aggressive fish. RTS have been known to live more than 12 years and reach 8-10". A 55gallon tank is the minimum recommended tank size.
My RTS and Clown Loach are best buddies...they share the cave that was once exclusive to the RTS. (He didn't used to let ANY other fish in that cave)... They are always side-by-side.