[name_m]Cosmo[/name_m] is such a cool name! I’ve heard of a YouTuber with a big family who has twins named [name_m]Cosmo[/name_m] and [name_f]Sylvie[/name_f], and I’ve always thought those names paired really well together.
I think all three of your options could work beautifully, just for different reasons. [name_f]Nova[/name_f] does feel a little more heavily tied to the celestial/space theme alongside [name_m]Cosmo[/name_m], so it depends on whether you like that association or would prefer something a bit more distinct. Personally, I think it could be quite cool and might even make the sibling names feel the most cohesive. I do agree with others that [name_f]Nova[/name_f] [name_f]Vivienne[/name_f] doesn’t have the smoothest flow because of the two V sounds so close together, but unless you’ll be saying her first and middle names together often, I don’t think it’s a huge issue.
[name_f]Tilda[/name_f] feels bright, fun and full of personality, which works really well with Cosmo’s quirkiness. Together, I think they have a lovely vintage charm without feeling too matchy.
[name_f]Allegra[/name_f] feels a little more grand and elegant in comparison, but I can still see it working well with [name_m]Cosmo[/name_m], depending on which aspects of his name you’re drawn to. I also love that [name_f]Allegra[/name_f] has a connection to your Italian heritage, which makes it feel especially meaningful.
Honestly, I don’t think you can go wrong with any of them! They each have a distinct feel while still working well alongside [name_m]Cosmo[/name_m]. My order of preference would be: [name_f]Tilda[/name_f], [name_f]Nova[/name_f], [name_f]Allegra[/name_f]