I would recommend taking the fish back.
Goldfish are temperate water fish (they enjoy cooler temperatures 60-low 70s)
Plecostomus are tropical (upper 70s to low 80s)
Crayfish are hostile and kill anything they can, and are usually prey of anything it can't kill.
Goldfish need atleast 20-30 gallon tank for one by itself, and 10-20 more gallons per goldfish after that.
plecostomus tank needs depend on the species, but the common plecostomus need a 75 gallon tank.
There is no advice on how to cycle the tank with those fish in it... those fish do not need to be in it, therefore if I were to advise you on how to cycle it anyway I would feel as though I were condoning that.
