Here are some general rules we use that might help simply which versions are compatible with others:
Components that share the same major/minor version are almost always compatible (for instance, all 7.0.9 agents are compatible with 7.0.7 relays).
The BigFix Server and Console are usually required to be upgraded at the same time.
We recommend you upgrade in the order of Server/Console -> Relay -> Agents.
Generally it is best to be on the latest version of BigFix if you want to take advantage of the latest features/bug fixes.
To specifically answer your question, you can deploy any version of 7.0.x for your agents… but note that we are just about to release a new patched version of 7.0.9 that fixes a few issues including an annoying custom sites issue: http://forum.bigfix.com/viewtopic.php?id=1881
So you might want to wait a couple days and then upgrade everything to the newest version.