Gliders will spend most of their time underneath cumulus clouds, if there are any. Fly in the blue bits between the clouds, and you minimise your chance of a close encounter. Fly above the cloud base, if you can - that'll keep you above 95% of gliders. Don't play too close to nice fluffy clouds - there might be a glider inside which could pop out of the side unexpectedly....
Alternatively, fly low. Away from gliding sites, you don't find many gliders below 2000ft. When soaring cross country, unless you're an expert that's the sort of height where you start accepting any kind of thermal, no matter how weak, just to stay in the air.
Also, when close to gliding sites, bear in mind that there will be more gliders upwind than downwind. The less experienced glider pilots don't stray too far downwind from home, in case they can't get back. And the more experienced ones don't stay close to home.
As for the rates of climb and descent - yes, they can be very high. When flying a high performance glider, the technique is to fly fast between thermals, then pull up and slow down when flying through them. The pull up is quite sharp, and the climb rate (for a few seconds) can be several thousand feet per minute. The rate of descent can be high too, but this happens much less often than it used to when I was an active glider pilot! My most shameful moment was beating the tug back to the ground from 2500 ft........
Keep your eyes open, and your wits about you and be a wee bit paranoid - but don't let it spoil your fun. The glider pilots will be looking hard for you, and aren't "out to get you"!
Final point - once you've qualified, go to a gliding club and have a go. After a couple of flights, you'll be able to predict their behaviour much better.