How can I skip the first month billing for a customer who buys hardware and an included subscription?

You can choose bill lines that should be skipped in the invoice based on business requirements.

The Subscription Management bill line object has an extensible field called Skip Flag that you can use to map any of the standard attributes or custom attributes in the subscription header, product, covered level or bill lines, to the service mapper and define the custom logic in the Subscription Management and Accounts Receivables Integration algorithm to skip sending bill lines to accounts receivables.

For more information, see Subscription Management and Account Receivables Integration Algorithm Customization White paper (Doc ID 2732008.1) on My Oracle Support.

Also, see Unable To Skip Bill Lines That Are Checked As 'Do not invoice' When Running The 'Send Billing Information To Receivables' ESS Job (Doc ID 3015024.1) on My Oracle Support in case you get an error.