I don't care for either name (even more so for siblings, because it would look theme-y).
Constantine is very feminine for a boy, to me. And, if associations bother you (they don't me), I think of
Keanu Reeves in the new movie
Constantine.
Conrad doesn't sound mature to me - wouldn't suit an adult, I think. Again, if associations both you, I think of the musical/movie
Bye Bye Birdie - the character
Conrad Birdie.
I don't think nicknames should be planned, but instead should be left up to the child. Anyway, I see
Connie as a feminine nickname, and Con as a short form of
convict (like the movie
Con Air, for example). But, if the child wants them as the nicknames, then so be it.
-
Lissa Hannah-
As soon as tradition has come to be recognized as tradition, it is dead. - Allan Bloom