Wiki says: Creatures and people up to level 6 will attack anything nearby for 30 seconds.

Great, how do i know what level my opponent is?

Never played as a mage before!

Basically, you don't! But you may learn to recognise higher level mobs.
So you will face instances of the spell reporting that the enemy is too high a level, only if you dual cast with all perks is it guaranteed. (edit: assuming you are not outlevelled too).
Not such a big deal with silent casting, and sneak, but otherwise could cause problems.
Illusion is still very strong though, but you do have to perk it or it is useless for those spells at least.

There's no enemy level indicator in game, so by trial and error the enemies above lvl6 will resist.

Or you could get the prima guide or look it up online.

For example, lvl6 animals are wolves, chickens and mudcrabs. Petty souls. Lvl6 people are bandit and bandit outlaws. Bandit thugs and all others are lvl9+ so by the time you're lvl 20 or so you won't find many lvl6- bandits b/c enemies scale with you.

Is there an equivalent fury poison?

It would be fun to shoot poisoned arrows at guards and watch them fight each other.

Yes, Frenzy. You can alchemy make it too.

I have been playing illusion magic a lot lately with my mage/assassin build. Honestly don't worry about what level the spell says it affects. Picking up the perks like animage and such that boost the levels that the spells can effect don't seem to change the spell description. Fury works on most creatures that you could handle in actual combat up until about level 12. Best thing is just trial and error. You will learn about where the spell starts becoming ineffective and then you will need the next level of the spell, like frenzy. It works well if you started using illusion magic from the start. If you are level 30 and just now are trying to cast fury for the first time... it just wont work.

Enemies level up with you, so it's a safe bet to assume that it will only work till you reach level 6 yourselve. There are more powerful version when you have the skill to use them and don't forget to pick the apropriate perks to improve them. They are also more powerful when you dual cast these type of spells.

Btw, fury on a chicken in town is hilarious.

question : is there always a way to make these illusion spells to work at any level, including perks related and dual-casting ? I mean, I have a level 55 character, and with all the perks, even the most powerful illusion spells seemed ineffective against the town guards. dual-casting is the only solution here ?