Tetras are a schooling fish that would do much better with a group of eight to twelve assuming the tank has cycled. I might were it me,, Consider asking a local fish store if they would accept the pleco you have if it is the common variety. These fish can and do grow upwards of eighteen inches or longer and as you have witnessed,, are serious waste producers.
I would look for a variety of pleco such as the bristlenose pleco that are much better suited for your tank and don't produce near the waste. The long strands of waste trailing from the pleco are normal. The larger the fish gets,, the more waste is produced. The bristlenose pleco rarely reaches six inches and is also a decent algae eater where the common pleco,, loses interest in algae as they grow larger and realize that fish food is much easier to come by than sucking on the glass for algae.