Hi Rohan !

Yes, you can use the certificate form government of bihar to prove your category , as you are using your delhi domicile , it is just to get a seat in delhi for 85% state quota. But this does not mention your category anywhere, so to prove your identity or infact category , you can use any offcial document by government of India. As government of bihar has issued it , no one can question it , because it's a state government , the document must have authenticity. So , there are almost no chances of rejection.

All the best !

https://www.mcc.nic.in/MCCUG_aiq/Show-Pdf?Type=50C9E8D5FC98727B4BBC93CF5D64A68DB647F04F&ID=F1ABD670358E036C31296E66B3B66C382AC00812 You can check here about the certificate. It is the official website link.