I believe Ramshorns can also do cold water
May even slow down the breeding too..... I was told that MTS snails breed more in warmer waters so maybe that might be another option...... I know I had some MTS in my pond snail bowl that seemed okay with no filter and no heater.
I would recommend trapdoor snails. The adults should be too big for the goldfish to eat. They are commonly kept in ponds with goldfish and such.