My mother has been prescribed several medicines, including this Vitamin B12, Niacin and Pyridoxine injection - is it safe to use generic versions for all of them?

Generic formulations that match the same salt and strength as what was prescribed are generally considered equally effective, so this isn't something to worry about across the board. It's still a good idea to run the full list past her doctor or pharmacist, though, since a couple of her medicines might be more sensitive to brand-to-brand variation than others. That way you can switch with confidence rather than guessing which ones are fine.