1. No, but it's impossible for you to know which ones, unless you're experienced keeping plecos and have learned to spot the small differences between the species.
2. Keeping the pleco separated from the goldfish, or by simply getting a kind that is generally a nice and/or social pleco. The smaller ones are usually better.
3. Common Ancistrus or Bristlenoses are a type I've kept with goldfish for 10 years or more, they're very kind fish and are effective eating algae. They won't be much bigger than 8-10 cm and they breed quite often, if you have a boy and a girl, there'll be some profit in bringing the babies to the local pet shop every once in a while