Can I Give My Dog generic methyl/cyanocobalamin?

Yes, with care
Vitamin B12 (oral cobalamin) is commonly referenced for dogs at published doses. Published guidance for dogs: small: 250 mcg; medium: 500 mcg; large: 1000 mcg daily, every 24h. Water-soluble, wide safety margin. GI-disease dogs often need vet-administered injections instead, oral absorption is the problem being treated.
Source: Veterinary Partner (VIN)
Get the exact range for your dog's weight

The safe habit

Confirm any medication decision with your veterinarian, who knows your dog's health history and other medications. VetDose shows published reference data, not prescriptions.