Straight away I’d say no to [name_f]Rosie[/name_f] and [name_f]Millie[/name_f] as they’re too ‘nicknamey’ next to [name_m]Simon[/name_m].
I’d also say that [name_f]Esme[/name_f] was too modern-sounding, and that [name_f]Fenella[/name_f] and [name_u]Adair[/name_u] don’t have the same ‘vibe’ as [name_m]Simon[/name_m].
My favourite out of the rest of the names is [name_f]Beatrix[/name_f], mainly because you could then have [name_u]Si[/name_u] and [name_f]Bea[/name_f], which I think is super cute 
Or, [name_m]Simon[/name_m] and [name_f]Lucy[/name_f] - so, [name_u]Si[/name_u] and [name_f]Lu[/name_f]!
[name_m]Simon[/name_m] and [name_f]Tabitha[/name_f] also works well together, in my opinion.
[name_m]Simon[/name_m] and [name_f]Clara[/name_f] / [name_m]Simon[/name_m] and [name_f]Alexandra[/name_f] are both nice but don’t stand out to me.