Of all these names, I like [name_m]Nestor[/name_m], [name_m]Clarence[/name_m], [name_m]Rufus[/name_m], [name_m]Sylvan[/name_m], and [name_m]Gustav[/name_m] (though [name_m]Gustave[/name_m] instead).
I would choose [name_m]Clarence[/name_m] [name_m]Nestor[/name_m] and [name_m]Sylvan[/name_m] [name_m]Gustave[/name_m]. [name_u]Clare[/name_u] or [name_u]Ren[/name_u] and [name_m]Sil[/name_m] or [name_u]Van[/name_u] for nns.
While I am fond of [name_m]Rufus[/name_m] (I know two epic cats named that!) it seems whimsical to be paired with [name_m]Sylvan[/name_m]. A bit the same with [name_m]Wesley[/name_m], which sounds to me stiff and preppy compared to the darling [name_m]Nestor[/name_m].