Rico, who will turn 19 on Thursday, was 0-1 with a 7.04 ERA in 11 games (nine starts) for the Dominican Pirates. Last year, he pitched for the Venezuelan Pirates after signing as a free agent.
Santos is from Bonao, Dominican Republic, and Rico is from Guanta, Venezuela.
Both Mazzaro and Robinson were designated for assignment by the Royals on Nov. 20.
Mazzaro, 26, was obtained by the Royals from the A's as a starter after the 2010 season in a deal for outfielder David DeJesus. He made the Royals' starting rotation out of Spring Training in 2011 as the fifth starter, but when sent to Triple-A Omaha for a tuneup, he was hit hard and lost the spot.
The right-hander had his problems with KC, most memorably on May 16, 2011, when he absorbed a 14-run beating in 2 1/3 innings of relief on a night the Royals were short of pitching. That made him the first pitcher since 1900 to give up that many runs in fewer than three innings. After the 19-1 loss, he was sent back to the Minors.
For two seasons, Mazzaro shifted between Triple-A Omaha and Kansas City, used both as a starter and reliever. For the Royals in those two seasons combined, he had a 5-4 record, a 6.72 ERA and pitched in 25 games, including 10 starts.
Robinson, 27, made a splash for Double-A Northwest Arkansas in 2010, when he won the Texas League Triple Crown with 29 homers, 98 RBIs and a .335 average. He had another big season in '11 for Omaha, with 23 homers, 100 RBIs and a .326 average.
But the left-handed slugger's path to the Majors was blocked by first baseman Eric Hosmer and designated hitter Billy Butler. So Robinson stayed with Omaha in 2012 and slipped to 13 homers, 67 RBIs and .292.
During Interleague Play this year, the Royals called up Robinson and he pinch-hit four times, including three at-bats at Pittsburgh, and went hitless. That's been his only Major League action.