Expanding on @lois6b's answer
Nails (Both finger/toe nails and hardware nails)
We go everywhere together.
Almost everyone has finger/toe nails and takes them with them
You bite me, but that doesn't mean that you're tough.
Can be bitten but nail-biting is synonymous with worrying, not exactly tough
You're not tough enough to eat me.
Most people wouldn't eat hardware nails. From OP; 'He looks like he ate nails for breakfast' is a phrase describing someone who looks tough.
Only the hardest are compared to I.
Consider the phrase 'Hard as nails' describing a tough person
I will bring about the end, in your place of final resting.
Consider the phrase 'Final nail in the coffin'
And with me as a resting place, you would not lie comfortably.
The 'lying on a bed of nails' feat, hardly considered comfortable
I'm known for success and perfection.
Consider the phrase 'Nailed it!', an utterance when succeeding with style
You'll go for my head, when you know what I am.
Consider the phrase 'Hit the nail on the head' both literally and figuratively.