System manager's create_column_family function should have a column_metadata keyword parameter.
This is by design. The additional complexity in allowing the specification of column_metadata when creating the column family isn't worth it, in my opinion, because it's simple to make those changes immediately after creating the CF using alter_column() and create_index() without any negative consequences.
I think schema updates are very expensive operation that try to avoid a RPC call to each new column justifies. Take a look at my patch to add column_metadata parameter as a dict mapping column names to column types. Its appended in my last pull request. If you like it merge it :)
Closing since this duplicates #111.
Allow specifying col validators in create_col_fam