Can I redeem all my iTunes gift cards at once?
Yes - stacking gift cards is not an issue. You will just carry a larger balance and less risk of losing the cash value if you lose the redemption codes or physical cards. 

You lose the flexibility to redeem them on another account. So if you’re not the family organizer, don’t redeem a balance or they will need to cut you from the family temporarily when you want to spend your credits. 

Gifts do not redeem twice and you cannot transfer a balance to another Apple ID or get a cash refund.