Hi Luke,
I’ve just took a quick look at “NP Quote Request WooCommerce” plugin. If I got it correctly, when customer requests a quote, then order with “Quote Request” status is created. If that’s correct – then yes, our plugin can do what you need. “Quote Request” status should be listed in our “Triggers” options, and you simply need to select it in “Decrease Stock > Triggers”.
As for multisite – no, “single site” license is enough.
Hope that helps. Please let me know if you have any questions.