Goldfish are non-aggressive fish, so putting other peaceful fish with them is fine. Guppies are warm-water fish but if your tank does not drop below 18 degrees Celsius (even at night in winter) then you can put them in, White Cloud Mountain minnows, Bronze shrubs, Shibunkins and Fantails (fancy goldfish).
Piranhas will eat your fish.
Neon Tetras require at least 20 degree water
catfish suck on your goldfishes scales
Clown fish are not only expensive but are salt water and goldfish are fresh water
Angle fish are tropical
I have 9 goldfish in my tank with 3 bronze Shrubs and 6 White Cloud Mountain Minnows, all are happy and healthy. I have had shibunkins, fantails and feeder goldfish in with them, at one stage I had two tadpoles (I think they may have eaten my Minnows) and a Catfish (Died because we had a power cut over winter) All were fine with water at 18 degrees.
I have owned gold fish successfully in a comunity tank for over a year.