What type of fish can I keep wih my goldfish that will help to clean the tank?

Corydoras are not algae eaters and will not keep the gl clean in any way. Plecos are great algae eaters in some cases, I don't know where you got your information. The problem with plecos is that they have a huge variety of species, and new ones are being discovered all the time. Some grow to 4' and some grow to about 1 1/4", which covers just about any sized tank you could own. Some don't eat algae at all and some will work better than windex (I'm kidding, don't use windex on your tank). For a tank that is 4' or shorter, a bristlenose L144 pleco will work wonders in most cases. Rubbernose plecos are another favorite algae eater. Both of these fish stop growing between 4-6". Otocinclus are great for 10 gallon tanks, because they only grow to 1 1/4" or so. Another option is the Chinese algae eater. They can grow to 7" or larger and are often aggressive when they are adults. Either way, you will need to suppliment their diet with algae wafers because they will not allow the algae to grow enough to keep them fed. Also, keep a piece of driftwood in the tank, they need it to properly digest their food. I hope this helps, email me if you have any questions.