Aw, how cute is she.
Lil(l)abelle is a lovely name, too, however you end up spelling it. I guess if you want to be sure it's pronounced properly by the vet and everyone, it'd be easier to spell it the way you say it?
I hope someone can help you with the food situation, because that's tricky and I'm not sure what would be best! I think feeding them separately, with little Lil(l)abelle having her junior pellets, and Mischief his adult pellets (either his current ones or Science Selective ones if you want to switch him to that), would be best, but I don't know how easy that would be to manage while they're bonding. Maybe just split them up for the few minutes it'll take them to eat the pellets? Put a barrier in the bonding pen between them so they can both eat their own pellets in peace, and so Mischief wouldn't eat Lil(l)abelle's after finishing his own? I think I personally wouldn't put Mischief on junior pellets if he's prone to putting on weight, but junior pellets for her would be good as she'll need the extra protein while growing.