To answer your first question, my harlequin will eat a WHOLE star (about 3-3.5 inches across, sometimes smaller). But other people's are different.
They can be fed (last without food) for about 3-4 weeks, but it is really best to feed it after its done with its star ASAP to make it very happy. -I feed mine withing a week after its done with it's star.
Don't bother acclimating the star, unless maybe theres a huge salinity difference. Otherwise theirs no need, its food. :lol: Out of the maybe 10 times I've fed mine so far (I've had it for about 3 months) none have died and they were never acclimated. I don't add another star to the tank until it's done with the first one completely.
There are many species they will eat. They prefer linkias over all stars, but go with chocolate chip stars and save your money. Most people use those because they are the cheapest.
And they will not eat brittle stars/serpant stars. They are too fast and scare the shrimp when/if they try to attack it.
Some people buy them (harlequin shrimp) to control populations of Asterina stars (small hitchiker grazer stars) and that works too, but they should be fed after all the stars are gone.
Many people use a special technique with feeding Chocolate chip stars to save money and time they'd spend going to the store every week.
They buy 4-6 stars, any size. And add them to a seperate tank/fuge/refugium. Every time the harlequin needs food, they cut off the leg of one star and feed it to them. The leg will grow back on the star, during this time of regeneration, you can rotate with the other 3-5 stars.
I, though, like to feed them whole stars just because its so much cooler watching them eat and attack it

:lol: And I think they enjoy it more too. And some people complain that their harlequin's will not eat star legs because they are not alive, but its all different.
Good luck! They are worth all this time and money, trust me. And a mated pair is best to have because its twice the beauty!