I provide some services at a flat rate and for others, I bill hourly. Can the ADR Notable Billing Module accommodate that?

Yes!  When you create a new billable, simply select a Custom Rate Type, enter the flat fee amount in the Rate box, and the number of times it should be applied in the Hours/Flat box.  You can allocate the costs among parties pro rata or customize the allocation just as you can with any other billable item. 

Read more in Managing Billable Items.