If I install a plugin for my server, can a reseller utilize this plugin as their user?
Munin allows this if you enable the "All" ACL.
JetBackup allows this within the Permissions Tab.
CloudLinux Manager or LVE for CloudLinux also needs full root privileges.
Please review the documentation for your specific plugins and verify you have the Reseller ACLs adequately configured. You can do this through the Edit Reseller Nameservers and privileges section of the WHM.