Can glutamates from natural sources be excitotoxic?

No: Excitotoxicity is the mechanism by which neurons die in pathologic processes like spinal cord injury, stroke, multiple sclerosis, als, brain trauma and stroke. Natural sources (for example, eating foods with glutamate) will not cause harm.

You flunked the quiz: Excitotoxicity is COMPLICATED. Don't assume you understand it just because you've read a few lines about it here or there. But that's not really relevant to your question. Glutamic acid (glutamate) is NOT an essential amino acid. Your body makes all the glutamate it needs from alanine &/or aspartic acid. Consuming "natural sources" of glutamate simply means your body needs to make less of it.