Generally, you can install a Cloud Control/Grid Control agent to monitor a database, which has DB Control configured. However, as database 8i is quite old, it is not certified for EM versions post 10.2.0.5. In addition, only DB 126.96.36.199 is certified for EM 10.2.0.5, and this EM version is no longer available for download from OTN.
If necessary, see MOS 1071023.1 on how to request for a media shipment.
I will not suggest to use older version of OEM to support 8i database. If you have majority of databases in 10G/11G , then you should use 12 c cloud control.
In past we used to monitor 8i databases with 10G Grid but you will not get all the features for 8i database.