Can I use my old 30 in Mac Pro monitor as the display for my new Mac Mini?

you can use the DVI input on the display.

you will need this adapter or something similar from another reseller.

https://www.apple.com/au/shop/product/MB570Z/B/mini-displayport-to-dvi-adapter

you will also need a DVI cable, provided you dont have either the adapter or the cable already

DVI Cables, Dual Link DVI Cables - Newegg.com

To get full resolution with the 30" display you need the DUAL-LINK DVI adapter.

https://www.apple.com/shop/product/MB571LL/A/mini-displayport-to-dual-link-dvi-a dapter