Fair enough, is there any obvious and well known ones.. like could I Nutpunch between raidens strings? or is generally only between a normal cancelled into a special.
Any d+1-special or d+3-special can be interrupted on block. With other normals it's just a matter of knowledge and I don't really know of any notable ones considering the characters I use. As you pointed out earlier, f+4-ground skull can be interrupted. I know if Jax cancels f+4 into overhead smash you can interrupt even without armour/invincibility (you can just d+1 or standing jab).
There are also gaps between strings yes. Raiden has gaps in the following:
b+3 (gap), 1 (gap), 2. In b+3, 1, 2, the second gap is easier I think. With 3, 3, 4 there's a gap between 3 and 4.