Hello,
If in the ad group, you have only ad customizers with default values and no other static ads, then yes, your ads for that ad group will be 100% from your customized ads.
In other words, the new feature enables you to fallback to default value instead of static ads.
About the 50% fallback, I doubt if this is the rule.
Under circumstances that your ads are shown, incidentally, there might be many times where your customized ads can't be shown, e.g., due to the space limit.
If you would like to know more about this, I recommend to ask experts in
AdWords Community. There are a lot of specialized people monitoring that forum as well.
Cheers,
Thanet, AdWords API Team