I think [name_m]Spencer[/name_m] is cute on a girl. For nicknames, I think [name_m]Spence[/name_m] is the most natural, but [name_f]Pen[/name_f] or [name_f]Penny[/name_f] would work.
Middle ideas:
[name_m]Spencer[/name_m] [name_f]Alina[/name_f]
[name_m]Spencer[/name_m] [name_f]Amalia[/name_f]
[name_m]Spencer[/name_m] [name_f]Anne[/name_f]
[name_m]Spencer[/name_m] [name_m]Ashley[/name_m]
[name_m]Spencer[/name_m] [name_f]Avery[/name_f]
[name_m]Spencer[/name_m] [name_f]Belle[/name_f]
[name_m]Spencer[/name_m] [name_f]Bethany[/name_f]
[name_m]Spencer[/name_m] [name_f]Caroline[/name_f]
[name_m]Spencer[/name_m] [name_f]Catherine[/name_f]
[name_m]Spencer[/name_m] [name_f]Charlotte[/name_f]
[name_m]Spencer[/name_m] [name_f]Christina[/name_f]
[name_m]Spencer[/name_m] [name_f]Claire[/name_f]
[name_m]Spencer[/name_m] [name_f]Emilia[/name_f]
[name_m]Spencer[/name_m] [name_f]Faye[/name_f]
[name_m]Spencer[/name_m] [name_f]Florence[/name_f]
[name_m]Spencer[/name_m] [name_f]Genevieve[/name_f]
[name_m]Spencer[/name_m] [name_f]Hannah[/name_f]
[name_m]Spencer[/name_m] [name_f]Jean[/name_f]
[name_m]Spencer[/name_m] [name_f]Juliette[/name_f]
[name_m]Spencer[/name_m] [name_f]Kate[/name_f]
[name_m]Spencer[/name_m] [name_f]Lena[/name_f]
[name_m]Spencer[/name_m] [name_f]Margaret[/name_f]
[name_m]Spencer[/name_m] [name_f]Marie[/name_f]
[name_m]Spencer[/name_m] [name_f]Maya[/name_f]
[name_m]Spencer[/name_m] [name_f]Mirabelle[/name_f]
[name_m]Spencer[/name_m] [name_f]Polly[/name_f]
[name_m]Spencer[/name_m] [name_f]Victoria[/name_f]
[name_m]Spencer[/name_m] [name_f]Zoe[/name_f]