normally the agents are backwards compatible. You just won't get the latest/added functionality that you would normally have with newer agents.
I can confirm that 8.3 is compatible with older versions of agent 8.1,8.2. However I am not sure about 7.6, because there is major change from 7.6 to 8.3.
I would recommend you to approach support team and get official confirmation.
The key word here is "normally." It is highly recommended that your agent versions match your BSA version. I have seen, for example, where the BSA version was 8.1 SP2 Patch3 and the Linux agent versions were 8.1 SP2 Patch1 (very similar) but the servers were not displaying all the server objects properly. We had to make them match in order for it to work.