For a fish that small, I wouldn't recommend going more then a few weeks without food.
Sometimes getting piranha to accept pellots can be a real challenge but it has been done many times.
I don't recommend feeding a diet of one item no matter what it is. Particularly a diet composed solely of shrimp is not a good idea.
Get a fillet of tilapia at the grocery store, cut it up into bite sized cubes. One fillet will last quite a while and will be cheap. Get a little tupperware container and throw them in there. I like to make rows and layer them with plasitc wrap just for the ease of getting individual pieces out. Throw it in the freezer and thaw pieces with warm water.
That along with the shrimp, but I'd suggest going the frozen route with that too, will be a good start.
I find that one trick is to get them eating a well varied diet for the best growth and color. Tilapia, Silversides, Krill, prawns, catfish, squid....I've even fed venison on many occasions.
Serra's can quickly become finicky eaters so it can be a headache.