I do prefer [name_m]Callahan[/name_m] [name_m]Jack[/name_m] but Zimmerman is intriguing, although I find all the Ms hard to say (probably just me).
[name_f][/name_f]Some middle ideas:
[name_f][/name_f]Zimmerman [name_m]Jasper[/name_m]
[name_f][/name_f]Zimmerman [name_m]Hugo[/name_m]
[name_f][/name_f]Zimmerman [name_m]Jack[/name_m]
[name_f][/name_f]Zimmerman [name_m]Rhett[/name_m]
[name_f][/name_f]Zimmerman [name_m]Felix[/name_m]
[name_f][/name_f]Zimmerman [name_m]Dexter[/name_m]
[name_f][/name_f]Zimmerman [name_m]Joel[/name_m]
[name_f][/name_f]Zimmerman [name_m]Rhys[/name_m]
[name_f][/name_f]Zimmerman [name_m]Axel[/name_m]
[name_f][/name_f]Zimmerman [name_m]Louis[/name_m]
[name_f][/name_f]Zimmerman [name_m]Kit[/name_m]
[name_m]Zebulon[/name_m], [name_m]Zenon[/name_m], [name_f]Zennor[/name_f], [name_m]Zimran[/name_m], [name_m]Ziggy[/name_m] might appeal too?