Depends on the size of the tank and the type of pleco you are thinking of. The common and the sailfin plecos grow very large (1-2 foot) so you need a pretty massive tank to accommodate one of those. A bristlenose plec (ancistrus) can fit into a 20-25 gallon tank, as long as you're prepared to do extra cleaning. A trio of otocinclus (these need to be in groups) could fit into a 10 gallon, but should not be put in a new tank as they are sensitive.
None of these should pose a threat to tetras and guppies.
Whatever you get, don't expect the catfish to do your tank cleaning for you. Any one of the fish I mentioned will eat algae, but what goes in must come out- expect to have to do more gravel vacuuming in return. Otos are not too messy, being small, but the other fish I mentioned do poop lots. There is no fish that will clean your tank for you, any more than you can buy a dog that does the gardening or a cat that clears the kitchen.