I have been rebuilding trannys for 30+ years and I have NEVER seen a C6 that was missing the #7 washer. Be it a car unit or a truck unit with bolt-on yoke and bearing in the ext housing.
ALL C6'S ALWAYS USE THE #7 WASHER!
Something is very wrong with your trans assembly and/or mis-matched parts.
You need to get someone with C6 experience to look at that unit. Or take it to someone that really knows C6's.
If I was there, I could figure it out in 10 min or less.
If you were to post good clear pictures of ALL the parts laid out on a white background I bet someone here could spot the mis-match.