You do not want to feed tigers hellgrammites(dobson fly larva). while doing some aquatic entomology with our local fly fishing club i found a hellgrammite and picked it up - ouch i wount be doing that again. it literally hung onto my finger while i tried to shake it off. Not to mention besides the mandibles, the shell is hard and the back of the head is like a rock, so i dont think they would be particularly good for digestion. In general, these are the following common aqautic insects that I would rule out for feeding sals/newts:
1. Hellgrammites
2. Dragonfly larva
3. Damsel fly larva
4. water boatmen/backswimmer
5. Water scorpion
granted there are others but I find these are some of the more common and easier to find "dangerous" insects.
I have fed my newts cranefly larva - these look like a big white worm, and i have fed them caddisfly larva after carefuly opening the caddis case.