MongoDB replica set: members with different versions of mongod The primary is running mongod 3.0.6 and the secondaries running mongod 3.0.7

is a mixing of versions allowed in the same replica set?

For upgrade it's best practise to not all nodes the same time.
For minor versions its not an issue, although i would suggest to keep the same minor version across all nodes to avoid different behavior (due to different bugs and optimization patches between versions) after a stepdown. For major versions is fine when the primary is one major version behind the secondaries. Its not a good idea to have a primary on different major version from a secondary.