Sgt. Rock stories seldomly have interactions with other mainline heroes, except in the biggest of crossovers. The only mainline non-crossover story that I can recall him interacting with a character would be his very short appearance in Up in the Sky.
It’s not like they’ll have him joining the JL. It’ll probably be a war film that has the DC logo at the beginning and otherwise has minimal reference to the bigger world it exists in. It likely will be similar to the approach taken by Marvel Studios in Werewolf by Night.
And I’m excited for that approach.