can we use a mongodb 3.0 and mongodb 2.6 in same replica set? i know that replicas using different storage engines can coexist in a replica set on MongoDB 3.0  but can we add a MongoDB 3.0 replica to a replica set that consists of MongoDB 2.6 instances ? 

That would be the easiest way to port to "MongoDB 3.0 + wiredTiger" instances from our MongoDB 2.6 instances
You can perform a rolling upgrade of your replica set from MongoDB 2.6 to 3.0.

While a mixed version 2.6/3.0 replica set is supported for the purposes of upgrading, you should not run with this configuration for longer than is required.

You should also read the 3.0 upgrade and compatibility change notes carefully, as important considerations will be included there. As with any upgrade, you should test in a development/staging environment before prior to your production release.

A few particular points to be aware of:


Before enabling the new WiredTiger storage engine, ensure that all replica set/sharded cluster members are running at least MongoDB version 2.6.8, and preferably version 3.0.0 or newer.
Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.5
If you upgrade to 3.0 and have run authSchemaUpgrade, you cannot downgrade to 2.6 without disabling --auth or restoring a pre-upgrade backup. The authSchemaUpgrade discards MONGODB-CR authentication credentials used in MongoDB 2.6.