Snails? A hillstream loach likes coldwater but prefers a strong current and I'm not sure how they would do with messy goldfish. They do clean the glass but only small amounts....so probably wouldn't solve the problem. Some people say chinese algae eaters can live in coldwater and make good algae eaters when young....but they stop eating algae with age and you may find them trying to suck on your goldfish.
If you have a bad algae problem your best bet would be to figure out whyand try to correct it.