What coldwater fish exactly? Goldfish? or something else?

Plecos are tropical fish, NOT coldwater, some will tolerate cooler temperatures but they’re best kept for tropical tanks, and well filtered ones at that as Plecos are horrendously messy animals, and there are over 800 species too. Snails will depend on what the fish you are keeping are, Goldfish will eat small snails and attempt to eat larger ones. Commonly sold Crabs are tropical animals and semi-terrestrial, as well as often needing brackish conditions. They’ll also eat whatever fish they can catch! There are freshwater fully aquatic crabs, but they’re not often seen in the trade, are often tiny, and would be eaten by larger fish.

So we need more details on your set up before we recommend anything!