When creating a new user with a order, how to email the new user login details
I have achieved this using Rules

The Rule is as follows.

Events
After updating an existing commerce order

Conditions
Data value is empty - commerce-order:owner:last-login

Actions
Send account e-mail

The reason why I have used this Event rather than 'After creating a new order' is to do with how this 'Commerce Admin Order Advanced' module works. You create the order before adding line items. And since I only need this email to be sent when a certain product type is created, this should suffice, hence I add the following condition

Order contains products of particular product types - Bespoke (an arbitrary product type)

But in order to prevent these emails being sent again should an admin update the order again, I have added the following extra Condition 

Data comparison - commerce-order:created == commerce-order-unchanged:changed