If your equipped weapon allows a re-roll, and it's all 3 red dice for example, do you have to keep track of which dice you chose as supplied by the weapon and re-roll that di only?
You may re-roll any dice of that color, not just the specific dice that were added as a result of using the weapon.
If a hero is surrounded by multiple enemies should their attacks and rolls be combined into a single roll if they are all a single group activated? Or are they activated 1 by 1 as single attacks even though there may be several surrounding the character they are attacking.
The enemies attack individually, not all at once. Note that all enemies in the activated group must finish their movement before the first enemy in that group attacks a hero. As soon as an enemy attacks a hero, or takes any other non-movement action, all models in the group lose their remaining movement points.
Same question for the heroes. If several heroes surround a single enemy should their attack be combined?
Heroes move and attack individually.