(I did misread part of this slightly earlier LOL whoops, the downside of being kinda distracted)
I don't think this would work unfortunately, the Atari were written to have kept a distance from RiverClan and vice-versa - they had very minimal interaction and a skirmish wouldn't have been feasible even with a purrk. The Atari were more concerned with the cougar hunting what few cats they have - they wouldn't care about the rando outsiders when they didn't have extensive resources to deal with it.
Also checked with taillow and they're hoping to use the Atari and RC in the future; they'd appreciate not generating random rivalries so future plots aren't heavily impacted by it !! @
Hushfalls: