I don't like Ernest/Ernie. I think it sounds too old and crusty for a little boy, and too silly for a grown man. I do like
Gideon, and I don't think it will get shortened in any serious way; if anyone calls him Gid or Giddy, it might be just his friends, in a casual way. I don't think he'll be
known as Gid/Giddy, if you call him
Gideon.
I advise you not to use a name you hate the shortened versions of, though. Once the kid gets old enough to decide for himself, he might like the shortened version, and then you'll hear it every day. You never know.
I think both
Ernest and
Gideon go well with
Theodora, but I would rather see
Theodora and
Gideon together as sibs. That's just me.
Hmm. Your ideas are intriguing to me and I wish to subscribe to your newsletter.