How can I specify a specific vm type for the cluster master (I don't want to use an high memory instance for relative an inactive node). Also, is there any way to add nodes to a cluster and choosing the type of vm? (this can solve the first problem)
Update November 2015:
Now that Google Container Engine is no longer in alpha, you don't need to worry about the size of your cluster master, as it is part of the managed service.
You can now easily add/remove nodes from your cluster through the cloud console UI but they will all be the same machine type that you originally choose for your cluster.
If you are running OSS Kubernetes on GCE, then you can set the MASTER_SIZE environment variable in cluster/gce/config-default.sh before creating your cluster.
If you are running on GKE, we unfortunately don't yet offer the option to customize the size of your master differently than the size of your nodes. We hope to offer more flexibility in cluster provisioning soon.
There is currently not a way to resize your cluster after you create it. I'm actually working on this for OSS Kubernetes in Issue #3168.