Hi Reefm...
Well, the critter you called a snail/slug was probably was Stomatella varia which is a grazer closely related to Astraea and Turbo, and as you indicate they are good grazers.
The presence of a proboscis is not always a good cue to predatory behavior. Queen conchs, Strombus gigas, are absolutely non-predatory and eat only algae, but have a long green proboscis. However, most of the time it is a good cue.
Now, we have to define things a bit better. The structure that looks like "snorkel" in front of the shell is not a proboscis. It is called a siphon and it only serves to direct water into the mantle cavity where the gills are - however, it has the snails equivalent of a nose at the base so it is used in predators to sniff out out food.
A lot of good snails have siphons, Ceriths, Nassarius, etc. So it can't be used as a character with any reliability.
Snail proboscides always are kept internally until the animal starts to feed and then are extended out of the mouth and used to attack the prey. Nassarius and Strombus will have proboscides, and are harmless to other tank inhabitants, but they are about the only ones.
Check out this article for more information about how to recognize good from bad in predatory snails.
http://www.animalnetwork.com/fish/aqfm/1998/oct/wb/default.asp
Cheers, Ron
[This message has been edited by rshimek (edited 01-16-2000).]