I love harlequin shrimp, I have one myself and love watching it take the chocolate star into its cave.
With that said... If that's all you want and you have no need for corals. A BC 14-29 would work just fine.
Or you could go cheaper and do a say a 20 long or a breeder add a AC110, heater and a powerhead. Get yourself some sand and some live rock and your set.
The all in one tanks like the BC has a good start but you'll still need a powerhead and heater.
It's really up to you and what you wanna spend.
I like the BC's I keep some chocolate stars in chamber 2 so when she comes out for dinner I have something to feed rather then head out to the store, plus if I buy more then one they usually give a deal.
Oh one more thing, if you don't go the all in one route you'll need to look into lighting also BUT with no corals in the tank the lighting won't be such a huge deal like it would of you had corals.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley