What does his shell look like?
He's most likely a Ramshorn snail: http://www.freewebs.com/stevenmattx2...rn%20Snail.jpg
But he also might be a Pond snail: http://www.bettatude.com/Snails/pond-snail1.jpg
Another "pest" species is the Malaysian Trumpet: http://www.aqualandpetsplus.com/Bug_S98.jpg
Pond snails will eat your plants but don't do much damage unless you have a huge number of them. Your guppies will probably be more dangerous for your plants, lol.
Rams occasionally munch on tender-leaved plants but usually they prefer rotting ones, algae, or leftover fish food. Rams don't normally cause any damage.
MTS (Malaysian Trumpet Snails) do not eat live plants...only dead or dying ones. These guys are pretty helpful as they stir up the gravel, eat algae and un-eaten food, and will consume dying leaves before they have a chance to decay. Supposedly they can sometimes clog filters in huge numbers but I've never had that happen. Oh, and they generally don't get out of control...if there's a big population boom it'll be followed by a die-off. Basically, if you don't over-feed your fish you shouldn't have a problem.
Actually, most "pest" snails won't get out of control if you don't over-feed. I wouldn't worry about the one you found just yet. I personally leave pest snails alone now. I haven't had any problems and they can be pretty handy. They'll keep algae off your glass, too. And they are a great live food source for puffers and loaches
And no, the snails won't bother your guppies.