If my metric is correct, 55 mm is just over 2 inches thick. That’s pretty good if it’s what you’ve got. The difference between 55 and 65 mm is not that great. My benchtop is 3 1/8 inches thick and it has been great. But if all you can get is 55 mm, then go for it. It’s possible that it will flex a little with heavy use, or that it might vibrate a little when pounding on a chisel. But if you chisel over a leg, it’s not a problem. The important thing is not to let these details get you off track. Build it. It will be fine for several years. If it proves inadequate later, make a new, thicker top.
Matt, Northern California - Started a blog in 2016: http://tinyshopww.blogspot.com/