After my opponent selects his defense tokens and I choose a critical effect (in this case ACM), what order does damage get dealt? I'm thinking if my opponent chose redirect does the damage from ACM get applied first thus limiting the number of hits that are redirected or does it wait as the last damage applied?

Critical Effects happen once you choose them know ACMs deal their damage before normal damage). Then you total damage (apply brace here), then you deal damage (redirect goes here)

There is no order of what damage comes first. Critical effects are not damage. They just trigger a critical effect.

Critical effects are seperate from attack damage (though they might deal additional damage as part of their effect) and are resolved after defense tokens are spent.

So in the case of assault concussion missiles, the extra damage on each facing cannot be redirected or braced.