For a tank that's only 20g, you could get away with about 10 pounds of rock. For the potential hassle of using used rock, you'd be much better off buying fresh cured rock from the LFS. You're looking at a difference of what, 50 bucks? One 4 week battle with algae problems from old PO4 soaked rock will quickly have you questioning the "deal' you got.
While I'm sure that you can find some rock less than a year old that someone is selling at a loss, it's more likely 3 or 4 yr old rock, and often the person has shut down or down sized due to issues with the tank, like algae problems due to high nutrients. Since the live rock is the foundation in which you're building your entire tank, is it really worth it to try and save 50 bucks?
Sure, you might get a crab in new rock. You might get it in old rock. So what, leave it or catch it, it's not tough. I once got isopods in a batch of new rock. They're bad. I caught them all in 20 minutes. Done.
Trust me, it's easier to catch one or two pests in new rock than to battle hair algae for 3 months on used rock. With a crab, it can be annoying. With algae problems, your tank will look like crap and you'll probably regret setting it up in the first place.
As for dry rock, sure, no issues, but yes, it can take a long time to become real live rock. Maybe a year. For 50 bucks??
For people starting a 300g tank, rock price can be a concern, but for a 20g, start right and your time in the hobby will be more enjoyable.
Just my opinion
