is anyone able to tell me if there's a mod that makes the summoned daedras agresssive and attacking ennemies at sight so it removes the necessity for the caster to be hurt by the monster he's protecting from? It would be pretty usefull for a squishy caster agains werewolves that can one shot him at all time

You don't need to get hurt, the summon just needs to observe an act of combat from either party. You can just throw/shoot something at your enemy from range.

Edit:
The easiest workaround would be to use a target spell with low magnitude and high area to make hitting your opponent as easy as possible. At a low mag. a warrior should be okay to cast it.

Does anyone know if it's possible to just put a script on summoned creatures to get them to target enemies on activate? I'm not sure if there a function to get enemies that are in combat.

http://www.fliggerty.com/phpBB3/viewtopic.php?f=46&t=1092 doesn't do that for standard summons, but it does have new forms of summoning which do.

Starting combat without the player being involved doesn't work so well due to AI limitations in the engine.