I wouldn't try that with the harlequin, Badfishofbabylon tried this for his harlequin and the starfish didn't make it after he cut off the first leg.
Try to see if you can get some asterina stars from your LFS
, that'd be the best way to feed one if you decided on getting one.
As for the live sand and magical cycle starter bottles, IMO
a lie and I wouldn't waste my money.
Adding established LR
you'll probably see little to no cycle.
And to reinforce what thom and TC said (Even though I know you've heard and heard this time and time again) the mandarin isn't a good idea. If eating prepared they can revert back to eating pods
in which case you would be using them faster than they can reproduce. JMO but heed their advice, it's not worth watching that mandarin starve so you can have him for a while
I'll stick around though just to see a nice pair of snowflakes or picassos, or s'more clowns!