I love this sort of thing so much! [name_m]Rosen[/name_m] as a nickname for Rosenkreuz is brilliant, honestly. It sounds so gentle and pretty on its own, and then the full name has such dramatic, old, slightly mystical energy. Very good “you are in trouble and I am using your full name” material.
A few ideas that came to mind:
[name_m]Tip[/name_m] — Tippetarius
[name_m]Pip[/name_m] — [name_m]Peregrine[/name_m]
[name_m]Kit[/name_m] — [name_m]Kittiwake[/name_m] or Kitteridge
[name_f]Nell[/name_f] — Penumbella
[name_f]Mina[/name_f] — [name_f]Wilhelmina[/name_f] or Minervina
[name_f]Rosie[/name_f] — Rosamundine
[name_f]Bee[/name_f] — Beatrixia or [name_f]Belphoebe[/name_f]
[name_f]Rue[/name_f] — Runeswith
[name_m]Toby[/name_m] — Tobias-Tybalt
[name_m]Finn[/name_m] — Finneganthony
[name_m]Dot[/name_m] — [name_f]Theodosia[/name_f]
[name_f]Lulu[/name_f] — Ludovica-Lunette
[name_m]Nico[/name_m] — [name_m]Nicodemus[/name_m]
[name_m]Moss[/name_m] — Mossifer
[name_f]Etta[/name_f] — Henrietta-Grimalkin
[name_m]Ren[/name_m] — Rensworth
[name_f]Bunny[/name_f] — Brunhildabeth
[name_f]Merry[/name_f] — [name_m]Meriadoc[/name_m]
[name_f]Wren[/name_f] — Wrenifred
[name_m]Ash[/name_m] — Ashurbanipal
I think my favourites are [name_f]Bee[/name_f] for [name_f]Belphoebe[/name_f], [name_m]Pip[/name_m] for [name_m]Peregrine[/name_m], and [name_f]Merry[/name_f] for [name_m]Meriadoc[/name_m]. They feel properly usable day to day, but the full names still have that lovely “sorry, your name is WHAT?” quality.