I have two of these algae eaters, one is quite big, about 4 inches, the other smaller. Whilst the big one is quite aggressive, it only picks on the smaller one, and leaves all other inhabitants alone. They both love the algae tablets.

I did have a goldfish tank a long time ago, and kept a couple of these algae eaters in that tank. They used to attach themselves to the goldfish and suck on them, but I never saw the type of injuries you describe.

They do have a good use though, they keep my glass pretty clean. When I do water changes, and the glass dries, I can see the marks their mouths make all over the glass.