I have always active, repeating command blocks that add "@e[type=skeletons]" to my team "Skeletons". This prevents the possibility of a skeleton aggro-ing on another skeleton as I have modified the team to prevent friendly fire. However, the team modification does not prevent the skeletons from doing damage to eachother with stray arrows. I have tried work arounds such as adding all arrows, something called "#minecraft:impact_projectiles" and "#minecraft:skeletons". None of this works and skeletons still do unwanted damage to eachother