"Cannot execute binary file" with alias of binary
Instead of using Finder aliases, use Terminal to create symbolic links (see man ln for details on usage):

ln -s /path/to/python3.3/binary /usr/local/bin/python
ln -s /path/to/python2.6/binary /usr/local/bin/python2.6


It's usually not a good idea to mess with /usr/bin content (will be wiped out with the next upgrade). Use /usr/local/bin instead and make sure it comes before /usr/bin in $PATH.