I'm a newbee, so take what I say with a grain of salt...
Personally I will NEVER get blue legs again. 20 blue legs managed to take out about 10-15 snails, about the same number of scarlets, and have worked themselves down to about 10. this is in a 40 gallon tank with about 50#s of LR. Its not uncoman for me to see 3 or 4 blue legs all fighting each other to get a shell. Also, they only seem to like FRESH shells, and size DOES NOT MATTER. I've seen blue legs kill a scarlet only to find that it's shell is about 75% SMALLER than his current shell. I've had to put some of my scarlets in my refugium to keep them alive! And yes, I have about 40 loose shells scattered around the tank, as well as more than enough food to keep everybody happy (both veg and meat).
Sorry if I sound a little 'intense' but I'm regretting ever getting them. I wish they had not been bunndled into the U-Build-It package I bought.