These are only some apple snails. Most snails actually will eat plants, but the most common kinds (Bridgesii) will not. They will if you offer them no other food. Other kinds of snails, like Canas, will eat plants regardless of the food you give them.
I can't give you a direct link, but on applesnail.net, go to "Care" then "Quick guide". It is, essentially, an awesome, quick little guide to the basics of snail care.
You will need to add calcium - try crushed coral as a substrate, or add Kent Marine Liquid Calcium daily, and/or use cuttlebone.
Goldfish all have different personalities, as I'm sure you've noticed. Some goldfish (like mine) will pick at snails, causing them to lose tentacles and other bad things. Some goldfish are fine. I would prefer not to risk it, but if it's your only option - add the snails and carefully observe the interaction between the goldies and the snails. If the goldies start picking on the snail(s), pull the snails out immediately.
Also snails like warmer temps than goldies do, as they live generally in South and Central America in the wild.
As to how many: the general rule of thumb is 2.5 gallons/snail, but that's only because they poop - A LOT. If you have good filtration, you can get by on less than one gallon per snail. That's my experience anyways. I currently have 17 baby Brigs and an adult snail in a 10 gallon and they're fine.
For a list of snail sellers go here:
http://free.hostultra.com/~applesnails/sellers.htmIf I were you, I'd definitely get them from an online seller versus a pet store, because usually at the pet stores they are not well cared for, not very cool colours, and not long lived (surprise surprise).
If you have any other specific questions don't hesitate to ask.
