Dojo's will play with the goldfish, they will swim next to them, eat particles that are on the fish, but yet I have seen one actually suck on the goldfish, Weather loaches are meant for cold waters, they naturally live in rivers and streams.
I have 5 Weather loachs with my adult goldfish and I have yet seen any harm done to them or the loachs....I have seen the loachs try to eat the food out of my goldfish's mouth now thats a funny thing to see