walkers have a bizarre effect on cube design since they're so good against both in and against slower decks, on balance i think they're good for aggro decks because the only reasonable way to fight them is to go under but after declaring "TNN is good for Merfolk in every possible way!!" and getting pyroblasted into oblivion yesterday, i am not so sure.
i think 15/450 is just about right (so 12 for wadds?), then i'd take care to vary their colors, functions, and CMCs. a walker section with wildspeaker, relentless, good ajani, better ajani, good elspeth, bw sorin is less interesting than one with wildspeaker, caller of beasts, bad ajani, 5-mana elspeth, big sorin.