+1 for Perocore, ive had 4 mystery snails live with my comet and shubunkin goldfish and the goldies don't bother them at all.
Your fancy goldfish should be fine with a mystery snail as they also do well in room temp water my tank lowest is 65 and they snails do fine.
Just keep in mind that every goldie is different and yours may pay it no mind, bother it or think its food. Mystery snails do also produce a lot of waste so keep that in mind if you get one.
There fun to have in the tank though :)
Also if I remember correctly you can have nerites in with goldfish as well.