One has to remember that in the areas of the amazon where a number of plecos exist there is little in the line of protein based foods. Aquatic insects and small inverts do not do well in the heavy current. Plecs are very adaptable to feeding on availiable food sources and one is the mucus coating of other fish. There is good anacdotal evidence of these fish doing this in the wild and plenty of evidence of these fish doing this in captivity. The closest taxonomic relitives of Loricariidae are the Trichomycteridae. There are several mucus feeding only trichomycterid catfishes. It really would not be a suprise to find that mucus feeding is a common feeding method in many Loricariidae catfishes. These fish are really not obligate herbivores.
However all is not lost while there is no guarantee some possible plec are listed below
Acanthodoras cataphractus-(4")
Amblydoras hancocki-(6")
Platydoras costatus-(8")
Ancistrus dolichopterus-(4.5")
Ancistrus adonis-(30+")
Ancistrus lamboensis-(6")
Panaque nigrolineatus-(9.5")
Panaque suttonorum-Blue-eyed plecostomus-(7")
Peckoltia vittata-(4-5")
Glyptoperichthys gibbiceps-(19")
Glyptoperichthys punctatus-Spotted sailfin pleco-(7")
Rineloricaria fallax-(4.5")
Chaetostoma sp.-Bulldog Catfish-(9.5")
Liposarcus anisitsi-Snow king sailfin pleco-(17")
Synodontis alberti-(6")
Synodontis angelicus-(22")
Hoplostermum thoracatum-(8")
Hemiancistrus annectens-(8")
Hemiancistrus landoni-(9")
Isorineloricaria spinosissima-Spiny suckermouth catfish-(12")
Loricaria nickernsis-Nickerie suckermouth-(6")
Baryancistrus sp.-(L18,L81,L177)-(6.5")
Leporacanthicus galaxias-Galactial suckermouth-(13")-Anne