I think that if you are allready at ,or over your max of Goldies,then it's not a good idea.They also have to be a certain size in comparison to your Goldies.If they can fit into a Goldfish mouth,they'll become snacks.
Snails are almost as messy as Goldfish,need a very good filtration system and a Ph on the higher side.
I'm going to move this to the snail section.