Question

Is an atom most stable when it has eight electrons in its outermost energy level?

Answer 1

Yes, if the outermost energy level is the combination of $s^2{p}^6$

If the outermost energy level is $s^2{d}^6$ it is not the most stable.

In the case of iron ${Fe}_{26}$. The $3d^6$ electrons could be considered to not be part of the outermost energy level because Iron is in the fourth energy level and the $3d$ electrons are part of the lower third energy level.

Practical however the $3d$ electrons are part of the valence electron structure of the transition metals and therefore are part of the outermost electron shell.

So it is important to specify that the stable octet is only for the $s^2{p}^6$ configuration.