On Thu, Aug 1, 2013 at 2:07 PM, Morgan Segalis <msegalis@gmail.com> wrote:
Actually I was thinking I would like Cassandra choose by itself the token.

You NEVER want Cassandra to choose its own token in production. There is no advantage to doing so and significant risk when used as a matter of course. The conf file even says you should manually specify tokens in production..
How to make this node know that it will be a Seed.

The only thing that makes a node a Seed is that any other node has it in its seed list. 

My current Node A is using Cassandra 1.1.0

You should not run 1.1.0, it contains significant and serious bugs. You should upgrade to the top of 1.1 series ASAP.
Is it compatible if I install a new node with Cassandra 1.2.8 ? or should I fetch 1.1.0 for Node B ?

It is not compatible, use 1.1.x with 1.1.x.