[name_m]Mickey[/name_m] has a really friendly, familiar feel to it, very soft and approachable. It does read as a nickname first (usually for [name_m]Michael[/name_m], [name_f]Michelle[/name_f], etc.), but it can work as a standalone too.
In terms of gender, I’d say it leans fairly unisex as a nickname, though in practice I still slightly associate it more with boys just because of [name_m]Mickey[/name_m] [name_m]Mouse[/name_m] and the traditional [name_m]Michael[/name_m] link. That said, I wouldn’t find it surprising on a girl either.
[name_m]The[/name_m] vibe for me is quite playful, a bit retro, and very warm/“everyday friendly” rather than formal. [name_m]Mick[/name_m] feels a bit sharper and more grown-up, which could be a nice contrast as a nickname option.
For sibling names, I think it works best with other classic, slightly vintage names or simple, unfussy choices, like [name_m]Leo[/name_m], [name_m]Jack[/name_m], [name_m]Theo[/name_m], [name_f]Ruby[/name_f], [name_f]Ella[/name_f], [name_f]Molly[/name_f], [name_m]Henry[/name_m], or [name_m]Sam[/name_m]. Names that feel easy-going and timeless seem to match its casual energy best.