Shoutouts to Volt , Pogo, and PKB for being my Costars :^)
Had them setup during a raid, Iron Defense, Belly Drum, and Nasty Plot, then had my Flamigo die. When it re entered, Costars did trigger and Copied the Iron Defense Buffs.
Conclusion, Costars does trigger but only copies one ally, it appears to be the most top person in order besides the user.
Hope this helped.