my gar and silver arrow did not play nice.. Gar won.. arrow is healing up well in a seperate tank.. Jars are notorious for going crazy at about 12" and killing their tank-mates. either Might work... But be prepared to seperate if need be if you try it. atm I keep 3 fire eels and a royal pleco w/ my florida gar and everyone plays nice. I would also have reservations about the RTC long-term.. it can potentially eat your gars.. and catfish are simply unpredictable ime.. personally I wouldn't chance it. and just about anything else you want to throw in will likely get eaten by the RTC eventually.
They say that plecos are not good with gars, because plecos are likely to fed with fish slime, then gars are slow moving fish and easy to the pleco to feed with their slime coating!!