A few tiny drops of cyanoacrylate glue, applied on the feed (with the tip of a toothpick) where you want to place the back part of the nib do the trick.
Please don't. It's tough enough to remove a properly set nib and feed from the NN/Balance/Connaisseur sections because of the tight fit. If the nib is loose, it's the wrong nib. Never, ever, use an adhesive to secure a nib in the section. You may never get it out again, not to mention that super glues break down in the presence of moisture (ink) and heat, so it would fail sooner rather than later.
The #5 nib is indeed too small. Picture below: Left is a #5 Feather Touch, right is the Balance II nib. The feed will be set at the same place under either nib, so it's the length at the tail that matters. The difference may not seem to be much, but the #5 will not reach down far enough for the nib to wedge tightly between the inside of the section and feed. Because a C/C section has a closed end with the piercing tube, you can not set the nib in farther to compensate.
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