I'm certain that Oracle 10g client will not allow any database previous to 8174 to be connected to. So I suspect that is why 8173 cannot be monitored using the 10 Grid as it will use a 10g client. Now, I have no experience of 10G Grid (yet) so I don't actually know if you need a 10g client, but if you can get away with a 9i client, that will connect down to 8173 and I think 8.0 as well. I do know that it doesn't talk down to 734 though :o) Hopefully the above is of some help, but if not, apologies for wasting your time.
The 10g Agent comes with all the necessary "client" utilities and libraries in its own home, so I do not think you can use another client install to over come not being able to connect to lower then 8.1.7.4.