I have always been fond of the common whiptail, Rineloricaria parva. They are fine as a solitary individual, or two or three. The length is 11-12 cm (5 inches max) but they are so narrow you really don't notice this, and their impact on the biological system is a fraction that of a BN or similar pleco. I think I've had one of these fish in a tank ever since I entered the hobby in the 1980's. The photo below is my present representative, who has entered its 12 year with me, pretty good for a fish with an average expected lifespan of 5-8 years. It's on the wood at the left, and second photo on the substrate. I also have (photo 3) one of the so-called Red Lizard Whiptails, though it is more brown than red, but I have never seen them interact at all even when they are next to each other. Avoid the "Royal" whiptails, that is a different genus and the fish are considerably larger.
There are three different species regularly offered as "Whiptail" Catfish--Rineloricaria fallax, R. lanceolata and R. parva--that have slight variations in pattern and the placement of the ventral scutes. It is also possible that some available fish may be hybrids between these three and other species. All have identical requirements in the aquarium.
Whiptails should be maintained in well-planted tanks having a small-sized gravel or sand substrate, with chunks of bogwood. Leaves can be laid on parts of the substrate. Overhead lighting should be minimal and/or diffused with floating plants. This catfish tends to remain on the substrate, but will also continually browse plant leaves, wood and rock as well as the substrate for algae and bits of food; they prefer to rest in wood crevices or under wood or rock outcrops. They do not eat plants.
Some authors have suggested moving this species (along with several others) into the genus Hemiloricaria; Planet Catfish has adopted this, but the species is taxonomically deemed valid as Rineloricaria.