Izuna - 3.5 - Pre-EMS Sasuke essentially.
Sakumo Hatake - 4.5, sannin level apparently.
Tobirama - 3.5.
Sasuke Sarutobi - Didn't hear much about him so I'll leave it alone.
Elder Son of SO6P - Didn't hear much about his powers but instead what he inherited.
Younger Son of SO6P - Same as older.
Kushina(lol) - 3, not sure what the lol was for she seemed pretty strong.
Fugaku - 2, the only fight with him involved we know of ended with him laying in a pool of his own blood with little difficulty.
Hiruzen(Prime) - 5, if Hiruzen was in his prime I feel he'd be just about the strongest in the manga.
Kushina and Fugaku would be the weakest. Elder & Younger Sons of SO6P would be the strongest.
Not necessarily, we only saw that they inherited part of their father's power and it seemed that this power was watered down significantly.