Running with es1.7 and es 2.1 in ONE cluster

i have ten nodes (elasticsearch-1.7.2, logstash-1.5.4, kibana-4.1.2-linux-x64) and it's running well.

Now i want to add some nodes with newest elk version (elasticsearch-2.1.1, logstash-2.1.1, kibana-4.3.1-linux-x64). so there will have multiple version es in ONE cluster.

Is this ok ? Will this have a problem?

That is not possible. All Elasticsearch nodes within a cluster need to be the same version.

You can run different versions if you are doing a rolling upgrade on the same major version.

But you cannot run different major versions like you are wanting to do. An upgrade from 1.X to 2.X is a complete cluster restart.