I wouldn't reccomend any type of apple snail in an 18 gal, maybe one tops, because they have a bad tendency to produce a TON of bacteria in their waste, and since bettas have such sensitive skin and fins, it could probably cause health problems. Snails aren't very efficient as algae eaters and often times like to munch on your plants instead. And yea, sometimes bettas will eat them. My betta had a snail with him but it produced a ton of waste so I took it out. He never tried to eat it, but I'd never trust a snail in with say, Shyboy, who has an amazing prey drive.
The plecs sound like a possibility, you'd probably only need one or two for an 18 gallon, depending on how many other fish you have (wouldn't want to get overstocked). I wish I knew what those little 2 inch algae eaters are... argh.