Daniel,
Yes, you pretty much should use the default service users that appear in the Managed Systems Configuration in SOLMAN_SETUP. If you are getting errors with the RFC connections, then you need to check that the service users are working (do this from SOLMAN_SETUP, not from the managed system). The tool will show if you there's a problem, such as the user being locked, or incorrect password, or if the roles need to be updated. Also, you can refresh the RFC connections completely from the tool.
No, client 000 connections are not mandatory. You can do this in your working client in each managed system.
Cheers,
Matt
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