This morning there was a game update to add Rex and Randall to the Premium Box! Until the end of the day November 4th there is "Lucky Time" for them so you have a greater chance of getting them from a Premium Box.
When they were added to the Japanese version Lightning McQueen and Tow Mater were also added along with a Cars Themed event. Hopefully the Cars Tsums will follow soon (along with the event).
Rex's skill is to clear a random Tsum Tsums. He stomps across the screen and clears Tsums as he stomps. His skill isn't bad but probably not as good as some of the other "burst" Tsums.
Randall's skill clears all the Randall Tsums currently on the screen and no more Randall's will appears for a period of time (4.5 - 6.8 seconds depending on skill level). His skill isn't that great but it is useful as another option to Zero and Pascal for some of the Bingo missions.