Welcome to AA!
Unfortunately the goldfish is unsuitable for a tropical fish tank and not in such a small tank. It is indeed a coldwater fish, will grow huge, produces copious amounts of waste, and has a high bio-load.
Also a common pleco is another fish unsuitable for such a small tank. They get very large and also have a high bio-load. A Bristlenose, Rubberlip, or Bulldog Pleco would work very nicely in this size tank. But no pleco should be added until the tank has begun to produce diatoms and algae as a pleco can starve if not given supplemental food in a new tank.
Since you already have 6 silver tipped tetra's you need to stop with them for now and do a fish in cycle. Here is a link you need to read and follow the instructions how to cycle your tank. After your tank is cycled you can talk about adding other fish....I just learned about cycling but I already have fish. What now?! - Aquarium Advice