purina flock raiser is fine for chicks too. B vitamins like vitamin C are water soluble so even though there’s more than what a chick requires any excess is easily purged from the body.
It’s generally not a good idea to raise goslings with chicks as the goslings grow much faster than chicks and bullying is a common result. It’s best to raise goslings with with other goslings.
I would strongly suggest finding another gosling for yours.
Also chick start is pretty bad for...