I don't think they are too close for brothers at all. I know a Dietrich who goes by both Dieter and Dietz. If I was gutsy enough to use a name like Dietrich, I wouldn't wimp out and use a Rick or Ricky nickname. :)
Do you shorten Cormac's name?
One momma namenerdin' in the fleeting quiet moments between
delighting in and losing my mind amidst my four kiddos
To peruse my top 100ish names:
http://www.behindthename.com/pnl/82589Bits of blogging here:
http://jodifriesen.blogspot.ca/And family cuteness here:
https://www.flickr.com/gp/41705