Traveller,
With the popular Sentiero Azzurro trails, there are guardrails etc. in a few spots but they are by no means "universal". On the trail from Monterosso to Vernazza, there are some narrow parts which don't have handrails, so you'd need to be cautious. If you decided to hike that trail, I'd recommend starting from the Monterosso end. Be sure to take lots of water. There's a steep climb at the beginning with some fairly high steps (again not many handrails) and I find that it's easier to manage the uphill portion first, rather than dealing with the downhill portion when I'm tired (that's also harder on the knees).
The trail segment from Vernazza to Corniglia is a bit easier, but again there are parts with no handrails. One thing to keep in mind is that thousands of people use those trails every year, with few problems. There's an extensive network of trails in that area, some of which go higher up in the hills, so they're a more robust climb. There's also a trail from Monterosso to Levanto, but I've never hiked that one.
As you may be aware, the Sentiero trail segments from Corniglia to Manarola and Manarola to Riomaggiore are currently closed, and will likely remain so for at least this year.