We have a clown pleco and an oto to take care of our 20g, along with a few ghost shrimp. Common plecos get HUGE and also (I've heard) dont eat algae their whole life, they grow out of it. The Clown, however, will eat algae its whole life and never get very big at all.
plecos do not belong in a tank with goldfish. they have different water requirements. Also..I have seen a large goldie with a perfect circle a pleco ate in it's side.
When I kept goldies, I had an apple snail in the tank. Did just fine keeping the algae down. the only thing you have to watch is if the goldies mistake the snails breathing apparatus for a worm and try to go after it. Mine diidn't tho