I have 2 scarletts in my 24 AP and they do a great job. I am fishless right now, but I have some nice patches of red gracila (sp?) and some other bigger red leafed algae, and they seem to munch on it a little, but once some hair algae popped up, they had it gone in less than 2 days.
They also seem to like to climb up the back wall, then fall to the sand. Last night they both took off up the wall like they were trying to see who could get up it the fastest!!! Too bad my astrea snail came along and knocked them both off.
My experience with blue legs are that they love to rip each other out of their shells and seem to play gladiator with your snails too. Jon