Hi,
Yes you can connect to a V8 Server using the V7 runtime client because to the best of my knowledge it would support forward compatibility . However I am not sure if it supports backward compatibility V8 runtime connecting to V7 Server due to the major changes at the server level.
However since there have been substantial changes between V7 and V8 IBM recommends that you use only V8 clients to connect to V8 servers as you might never know what operation fails