This is a fact seen in practice. You can check it if you want. Just create Google Apps account and configure mail without setting any MX records in your DNS. You will be able to send and receive emails to and from that account and regular Gmail account.
Not a complete counter example, but on a domain I run with external mail, gmail enabled but MX never setup, sending a email with google apps gmail, from me@mydomain to you@mydomain sends the email to the external provider
This is a fact seen in practice. You can check it if you want. Just create Google Apps account and configure mail without setting any MX records in your DNS. You will be able to send and receive emails to and from that account and regular Gmail account.