Maybe a cast only consisted of "Sato+Sere and their mons" would last, at its best, 1 and half episode. Reason? Yuuji Ueda (Takeshi's VA) is voicing Gekogashira and Sonans, so while Citron and Eureka are getting prepared for the 5th gym, pretty much Takeshi would have a brief return to Satoshi's traveling companions. Citron and Eureka would return to main cast after Miare gym battle because of them wanting to travel with Satoshi until the end of Kalos league (they expressed this desire to their dad on episode 9 IIRC)