you do not need a ledger if you are using hangers.So Does the length of the nails going through the existing rim board which I will be attaching the joist to using joist hangers matter and should I use a ledger board under the hangers?
are you talking about the joist hanger nails ?
if so, you have 1 1/2 " nails thru the flanges
the sides must be 2 1/2" nails IF they are "double shear" hangers.
double shear hangers will it stamped into the metal saying so
you need these longer nails in the side because they extend into the band board or header because there are fewer nails in the flanges.
I have seen many builders just use all 1 1/2 inch nails and the instectors never check.