Yes, they are catfishes (Siluriformes). They are then split into families (and then genus) like Calichthyidae (Corydoras etc), Mochokidae (Synodontis etc) and Loricariidae (Ancistrus, Peckoltia, Panaque etc) which are our 'plecos'.
What is the 'true species' you mention? Hypostomus plecostomus? There's something like over 700 described species of Loricariids (so far) and certainly hundreds more we know of and those yet to be discovered. I think they represent one of the most specially diverse families of vertebrate.