I'd say once their full sized with all their feathers. It sounds like Hollands get pretty big, so you can always supplement with Dubia roaches (for protein) or mix game feed half & half with layer feed. The biggest issue is just to make sure they have enough protein. If you want to raise them to lay, then mixing half & half would be fine, but if you want the toms ready for butchering, feeding them straight game bird will make the best growth rate.
But the biggest would be fully feathered, since that takes a lot of protein. So long as meat isn't your factor, then you'll be fine.
Also, chickens can eat gamebird fine. When we had turkeys we would simply switch out what we were feeding daily.. One day laying feed, the other day gamebird feed.
Also! Turkeys are tall. you can use this to your advantage, and make an area the chickens can't fly to, but the turkeys can reach to (though they might not want to bother).
Or you can do separate feedings (we did this when we had a lone turkey, we'd let him eat gamebird food whenever we were out there, since he always seemed hungry. But this wouldn't be too practical for multiple animals)
