I like both names!
I prefer
Ernest to
Ernie. I still have
Bert and
Ernie associations from Sesame Street although I'm sure if I knew an
Ernie IRL it'd grow on me.
I know a
Gideon and he's always gone by his full name. I don't think nicknames are inevitable.
For pairings, I'd rank them:
1.
Theodora &
Ernest2.
Theodora &
Gideon3.
Theodora &
Ernie (if
Theodora had a nn they might match better)