Hello eric_15,
Just to clarify, it seems like you want to know if an iMessage will be sent with SMS/MMS if Wi-Fi is unavailable, is that correct?
"To use SMS/MMS on an iPhone, you need a text-messaging plan. Contact your wireless carrier for more information. You can also set up your other Apple devices to send and receive messages from any Apple device.
If Wi-Fi is unavailable, iMessages will be sent over cellular data. Cellular data rates might apply."
Please check out this link which explains the difference between iMessage and SMS/MMS, and may be helpful: What is the difference between iMessage and SMS/MMS?
For more information on sending and receiving messages and the available options, please check out this link, which has some additional information that may help: Send and receive messages on iPhone
Thanks!