10.x agent on 12.1 engine:
not sure if officially supported, but it seems to work.
12.1 agent on 10.x engine:
don't even try ...
The agent will try to connect to all of your CP, then die:
20180122/135044.061 - U02000066 Host information: Host name='suicidal-agent.example.com', IP address='10.0.0.1'
20180122/135044.255 - U01002409 Message size is exceeded '1596' > '1572', module 'EXSTRTQ2.ip_list'.
20180122/135044.255 - U02000010 Connection to Server '*SERVER(ID=32749)' terminated.